Development of the Chemistry of Indium in Formal Oxidation States

Chem. Rev. 2007, 107, 2−45

2

Development of the Chemistry of Indium in Formal Oxidation States Lower than +3† Jennifer A. J. Pardoe*,§ and Anthony J. Downs*,‡ Department of Chemistry, Dainton Building, University of Sheffield, Sheffield S3 7HF, U.K., and Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QR, U.K. Received June 19, 2006

Contents 1. Introduction 2. Indium(0) 2.1. Indium Carbonyls 2.2. Other Adducts of Indium Atoms 2.3. Naked Metal Clusters 3. Indium(I) 3.1. General Characteristics and Subdivisions 3.2. Molecular Indium(I) Compounds 3.2.1. Monomers 3.2.2. Oligomers or Weakly Bound Polymers 3.2.3. Reactions of Molecular Indium(I) Compounds 3.3. Indium(I) Compounds with Appreciable Ionic Character 3.3.1. Preamble 3.3.2. Indium(I) Halides 3.3.3. Other Indium(I) Compounds 4. Indium(II) 4.1. General Characteristics and Subdivisions 4.2. Mononuclear InII-Centered Molecules 4.3. Compounds Featuring an InII−InII Bond 4.3.1. Formation of the In−In Bond 4.3.2. Preparation and Physical Properties of InII−InII-Bonded Compounds 4.3.3. Reactions of InII−InII-Bonded Compounds 5. Mixed or Intermediate Valence Indium Compounds 5.1. Preamble 5.2. Compounds with Discrete In or In2 Centers 5.2.1. Halides 5.2.2. Chalcogenides 5.2.3. Oxysalts 5.3. Homonuclear or Heteronuclear Cationic In Clusters 5.4. Homonuclear or Heteronuclear Neutral or Anionic Clusters 5.4.1. Homonuclear Clusters 5.4.2. Heteronuclear Clusters 6. Conclusions †

2 6 6 7 7 8 8 9 9 12 14 16 16 18 22 22 22 23 24 24 24 28 29 29 30 30 33 33 33 34 35 36 38

Dedicated out of friendship and scientific admiration to Professor Hansgeorg Schno¨ckel on the occasion of his 65th birthday. * To whom correspondence should be addressed. Tel: 0044-(0)1142229488. Fax: 0044-(0)114-2229346. E-mail: [email protected] (J.A.J.P.). Tel: 0044-(0)1865-272600. Fax: 0044-(0)1865-272690. Email: [email protected] (A.J.D.). § University of Sheffield. ‡ University of Oxford.

7. Acknowledgment 8. References

39 39

1. Introduction Many references to low oxidation states of the main group metals (i.e., lower than +n, where n is the number of valence electrons in the neutral atom) are apt now to signal metal clusters.1-4 Certainly cluster chemistry has developed far and sometimes furiously since Cotton first defined the metal cluster and likened cluster chemists to lucky collectors of lepidoptera.5 With no group of elements has the advance been faster and more dramatic than with the p-block metals of groups 13 and 14.1-4,6-9 The poetry of motion! The real way to traVel! The only way to traVel! Here today - in next week tomorrow! Villages skipped, towns and cities jumped - always somebody else’s horizons! Kenneth Grahame, The Wind in the Willows And no better sign of that advance has come than with the results conjured within the past decade by Schno¨ckel and his group at Karlsruhe6-9 and witnessed by the synthesis and characterization of such exotic species as [Al50(η5-C5Me5)12] (1),10 [Al77{N(SiMe3)2}20]2- (2),11 [Ga22R8] [R ) Si(SiMe3)3, Ge(SiMe3)3, or SitBu3] (3a),12 [Ga22(PtBu2)12] (3b),13 [Ga22{N(SiMe3)2}10]2- (3c),14 [Ga22{N(SiMe3)2}10Br10+x]n- (x ) 1, n ) 3; x ) 2, n ) 2) (3d),15 [Ga51(PtBu2)14Br6]3- (4),16 [Ga84{N(SiMe3)2}20]n- (n ) 3, 4) (5),17 and [Ge9{Si(SiMe3)3}3]- (6)18 (see Figure 1). It is through the intercession of bulky ligands to control the disproportionation of a univalent derivative of the metal that these ligand-sheathed clusters are formed. The term “metalloid” cluster6-8 has been coined to describe the condition in which the number of direct metal-metal bonds exceeds the number of metalligand bonds. Such stirring discoveries of main group metal clusters can be regarded as having their origins in two important, but apparently unrelated, developments in inorganic chemistry that occurred in the 1930s. On the one hand, they hark back to boron hydride1,2,4,19-22 and boron halide23 clusters, while contrasting with the surprising structures recently disclosed for B8F12 and B10F12.24 On the other, they relate to the interconnected homo- and heteroatomic anionic clusters that feature in intermetallic Zintl phases;1-4,25-30 examples include [Ga21]n-,25 [In11]7-,25 the triply fused icosahedral [In28]n-,31 [In4Bi5]10-,32 [Ga4Ge6]4-,33 and icosahedral [Cu6Ga6]n+ and icosioctahedral [MgGa16]n+.34 The newly discovered, ligandprotected metal clusters are noteworthy for the special

10.1021/cr068027+ CCC: $65.00 © 2007 American Chemical Society Published on Web 01/10/2007

Development of the Chemistry of Indium

Jennifer Pardoe studied Chemistry at the University of Bristol and was awarded her B.Sc. in 1998 and her Ph.D. in 2002, studying polyboron halide clusters under the supervision of Peter Timms and Nick Norman. After a stint as a postdoctoral research fellow with David Cole-Hamilton at the Unversity of St. Andrews, working with precursors for mercury cadmium telluride semiconductor devices, she returned to group 13 in 2003 to work on compounds in which indium is in a low formal oxidation state, this with Tony Downs at the Inorganic Chemistry Laboratory of the University of Oxford. Since 2005, she has been employed as a Teaching Fellow in the Department of Chemistry at the University of Sheffield. Her research interests have focused particularly on co-condensation methods as a means of studying the chemistry of reactive molecules such as BF, BCl, and InCl.

Tony Downs gained his first degree and Ph.D. from the University of Cambridge, U.K., where his research, concerned with perfluoroorgano derivatives of sulfur, was supervised by the late Professor H. J. Emeleús and by Dr. (now Professor) E. A. V. Ebsworth. Prior to his move to Oxford, he held a Salters’ Fellowship (1961−1962) at Cambridge, before being appointed a Senior Demonstrator (1962−1963), then Lecturer (1963− 1966), in inorganic chemistry at the University of Newcastle upon Tyne. At the University of Oxford, he was appointed first (1966) a Senior Research Officer, then (in 1971) a Lecturer, and later (1996) a Professor in inorganic chemistry, up to the time of his formal retirement in 2003; he has been concurrently a Tutorial Fellow of Jesus College. His research interests have focused mainly on reactive intermediates in the shape of hydrido and organo derivatives of both typical and transition elements. Characteristic of this research has been the alliance of synthetic studies (often requiring peculiarly rigorous exclusion of impurities) with a variety of physical techniques, including matrix isolation, vibrational spectroscopy, and electron, as well as X-ray, diffraction.

physical and chemical properties conferred upon them by their transitional nature in the region between isolated atoms or small molecules and the bulk metals. With dimensions of nanometer proportions, some of them hold particular interest in the burgeoning field of nanotechnology. They contrast with ligand-free metal clusters, which are much less amenable to experimental investigation and scarcely easier to model by quantum chemical methods.3,4,35-37

Chemical Reviews, 2007, Vol. 107, No. 1 3

Of clusters such as [Al77{N(SiMe3)2}20]2- there was barely a hint in 1993, a time of stocktaking as represented by two books treating many aspects of group 13 metal chemistry, one concerned exclusively with aluminum38 and the other with aluminum, gallium, indium, and thallium.25 The latter reviewed inter alia the lower oxidation states of indium, a theme also pursued elsewhere at about this time in reviews by Tuck.39-41 Here the emphasis was on halide and cyclopentadienide derivatives of indium(I), on the dichotomy for indium(II) between dimerization to form an In-In bond and disproportionation into In(I) and In(III), for example, In+[InI4]-, and on the redox reactions in which mononuclear In(II) species probably feature as short-lived intermediates. Metal-metal-bonded species such as M2[Ga2Cl6] (M ) Li, Me4N, or Ph3PH), Ga2Br4L2 (L ) dioxane or pyridine), and R2MMR2 [M ) Al, Ga, or In; R ) CH(SiMe3)2], all long-lived at ambient temperatures,25 were on the scene at this time. There was then a dawning recognition of bulk as a property of the supporting ligands crucial to manipulating reactivity at the metal center and to stabilizing previously unknown bonding types, geometries, or electron configurations. In the past decade, ligands have developed abundantly in both bulk and specific design in what has been a defining feature in the growth of our knowledge of the heavier group 13 elements in oxidation states between 0 and +2. Such ligands include substituted cyclopentadienyl groups (e.g., C5Me5),42 supersilyl groups (e.g., -SitBu3),43 substituted aryl groups (e.g., -C6H2-2,4,6-tBu3), terphenyl and related ligands,44 β-diketiminate derivatives,45,46 substituted diazabutadiene derivatives,47 and poly(pyrazolyl)borate groups.48 Their mediation has produced an outburst of new compounds containing metal-metal bonds, ranging in length from the deceptively short {in the compound Na2[Ga2Trip2] (Trip ) C6H2-2,4,6-iPr3) superficially analogous to an alkyne};49 through what may be regarded as normal 2c-2e bonds in the formally M(II) species R2MMR2;50-53 to the long, weak bonds in the M(I) clusters [RM]n.54-56 The extent of homonuclear multiple bonding in a compound such as Na2[Ga2Trip2] has sparked a lively and sometimes heated debate57-64 in which it is doubtful whether the final word has been said. At the other extreme, [RM]n clusters are typically tetrahedral (i.e., n ) 4)54-56 but, depending on the nature of the substituent R, may assume different forms, for example, near-octahedral [(η5-C5Me5)M]6 for M ) Ga65 or In,66 and [GatBu]9 with a structure analogous to that of B9Cl9.67 Irrespective of the nuclearity and shape of the cluster, interactions between the formally closed-shell (ns2) M(I) centers68 are often so weak that dissolution or vaporization results in dissociation to the monomers, although the ease of dissociation depends strongly on the electronic and steric properties of the substituent R.54-56,58 The highly reactive monomers, which can be trapped by cycloaddition reactions, show their carbene-like character in functioning as bases in compounds such as (η5-C5Me5)Ga‚GaCl2(η1-C5Me5)69 or, more strikingly, through coordination to transitionmetal centers, as in Ar*GaFe(CO)4,70 Ar*InMn(η5-C5H5)(CO)2,71 and Ni[MC(SiMe3)3]4 (M ) Ga or In)72 (Ar* ) C6H3-2,6-Trip2 where Trip ) C6H2-2,4,6-iPr3). As exemplified by the case of Pd3(AlCp*)2(µ2-AlCp*)2(µ3-AlCp*)2,73 such coordination may involve bridging as well as terminal functions.74,75 The bonding in these transition metal derivatives4,54,70-79 has also created quite an academic furor, again mainly with regard to the extent and relative importance of σ- and π-type contributions. Both experimental and quantum

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Pardoe and Downs

Figure 1. Examples of some metalloid clusters: 1, arrangement of the 50 Al atoms and 12 Cp* moieties of Al50Cp*12 showing successive shells of 12 ligand-bearing Al atoms (blue), 30 Al atoms (orange), and the central core of 8 Al atoms (blue); 2, arrangement of the 77 Al atoms in the anion [Al77{N(SiMe3)2}20]2- showing the outer shell of 20 ligand-bearing Al atoms (blue), the second shell of 44 Al atoms (metallic-gray), and central core of a single Al atom surrounded by a distorted cuboctahedral/icosahedral array of 12 Al atoms; 3, four different arrangements of 22 Ga atoms in the cluster species (a) Ga22R8 [R ) Si(SiMe3)3, Ge(SiMe3)3, or SitBu3], (b) [Ga22R10]2- [R ) N(SiMe3)2] (N atoms pink), (c) [Ga22{N(SiMe3)2}10Br10]2- (Br atoms green, N atoms pink), and (d) Ga22R12 (R ) PtBu2, P atoms purple); 4, [Ga51(PtBu2)14Br6]3- (Br atoms green, terminal and bridging P atoms blue); 5, [Ga84R20]4- [R ) N(SiMe3)2, N atoms pink] with its 20 ligand-bearing Ga atoms (blue); 6, [Ge9R3]- [R ) Si(SiMe3)3, methyl groups omitted for clarity]. (Structures 1, 2, 3, 4, and 5 reprinted with permission from ref 6c; copyright 2005 Royal Society of Chemistry. Structure 6 reprinted with permission from ref 18a; copyright 2003 Wiley-VCH.)

chemical results appear, however, to be at one in representing the very recently reported cationic iron complex [{(η5-C5Me5)Fe(CO)2}2M]+[BArf4]- [M ) Ga or In; Arf ) C6F5], with the structure 7, as having a linear Fe-M-Fe skeleton

in which there is appreciable multiple bonding.79 Here, however, we are no longer dealing with a group 13 metal in a low oxidation state, since the M atom is most plausibly assigned its normal (+3) oxidation state; rather is it the unsaturation of its remarkable two-coordinate environment that commands attention. Aside from acting as a ready source of the diyl monomer RM, [RM]n clusters display a rich chemistry in their own right, despite the constraints imposed by the ligand R.4,54-56 Simple mononuclear derivatives of the group 13 metals M in the oxidation state +1 and authentic paramagnetic derivatives of MII, that is, ‚MXY where X, Y ) H, halogen, or CH3, are the stuff of vapors and high-energy conditions. Otherwise they occur only transiently before disproportionating or aggregating, so detection and characterization require spectroscopic interrogation of either the vapor itself or of

vapor species trapped in a solid inert matrix at low temperatures.25,80 Thus, monohydrides and -halides are known for all the group 13 metals with ground-state properties well defined on the evidence of high-resolution electronic, vibrational, or rotational spectra of the gaseous diatomic molecules.25,81 Similarly, the monomethyl compound CH3Al has been identified by its pure rotational spectrum and by neutralization-reionization mass spectrometry (NRMS), with some reason to suggest that it acts as a carrier of the metal in planetary atmospheres.82 Gaseous AlH2 is a rare example of a genuine divalent aluminum compound that has been at least partially characterized by its electronic spectrum measured in absorption at high resolution.83 The lifetimes of such molecules can be extended indefinitely by matrix isolation,84 by which means it has been possible to delineate not only their physical properties but also their chemical behavior. For example, the reactions of AlCl set out in Figure 2 have all been established following co-condensation of the relevant reagents with an excess of a noble gas (usually argon); activation is brought about either thermally or, more often, photolytically, and the identities of the products have been elicited mainly from their IR spectra, supported by the effects of isotopic change and by appropriate quantum chemical calculations. The innate unsaturation of AlCl causes addition to be the favored mode of reaction, commonly with insertion into a bond of the reagent molecule, for example, H2, HCl, CH4, or HCCH. In the absence of a reagent, oligomerization may set in to give initially the cyclic dimer Al(µ-Cl)2Al and then the trimer, which is believed to have


Figure 2. Reactions of matrix-isolated AlCl molecules brought about by photoactivation (redrawn from ref 85).

Figure 3. Some reactions of matrix-isolated Ga atoms (redrawn from ref 86).

the unusual structure 8 featuring direct metal-metal bonds.85

Access to compounds of group 13 metals in the divalent state has been gained by similar experiments in which the metal atoms have been co-deposited with a potential reagent.86 The results of matrix experiments involving Ga atoms, summarized schematically in Figure 3, reveal that photoactivation is usually necessary to induce anything beyond formation of an adduct, but then reaction often proceeds through insertion of the metal atom into a bond of the reagent to give products such as HGaH and CH3GaH. These GaII molecules are distinguished by angular skeletons, and electron paramagnetic resonance (EPR) measurements have verified that the unpaired electron is indeed localized mainly on the Ga atom.87 Identification of adducts


of Ga atoms in their 2P ground electronic state with a variety of molecules, for example, N2, CO, C2H2, C2H4, H2O, NH3, and PH3, even opens up the zero oxidation state. According to density functional theory (DFT) calculations, the binding energies of some of the 1:1 adducts range from 8.4 kJ mol-1 for Ga‚N2 to 61 kJ mol-1 for Ga‚CO.88 With a total binding energy of ca. 125 kJ mol-1 for Ga(CO)2,89 it is thus evident that CO binds quite strongly to the metal, as attested by a mean ν(CO) wavenumber that is appreciably lower for Ga(CO)2 (ca. 1970 cm-1) than for Ni(CO)4 (ca. 2076 cm-1). The zero oxidation state also manifests itself in the simplest naked gallium cluster Ga2, which has been identified by a combination of matrix experiments and complete active space self-consistent field (CASSCF) calculations as having a 3Πu ground electronic state and a dissociation energy De e 145 kJ mol-1.90 This dimer is remarkable for being quite unlike Ga 2P atoms in reacting spontaneously with H2 at 12 K to give the cyclic hydride Ga(µ-H)2Ga.91,92 It is, however, the lower oxidation states of aluminum and gallium that have captured the eye, and the imagination, in so much of the pioneering research of the past decade. Despite a superior predisposition to reduction of the +3 state, as exemplified by the relative stability of the monohalides at room temperature,25 indium has often played no more than a supporting role.4,54-56 In part, this must reflect a lack of the sort of focused effort that the lighter metals have attracted. In the absence of a significant and consistent body of evidence, no firm conclusions can yet be drawn, but indium is unlikely always to follow the example of gallium, any more than gallium follows the example of aluminum. Indeed, both thermodynamic and kinetic differences are to be expected, for example, as a consequence of the larger size of the In atom and of the weaker bonds that it forms.25 Certainly there is no want of interest in indium chemistry to urge exploration of this area. Much of that interest stems from the applications indium finds in III-V semiconductor compounds25,93-95 that range from wide band gap nitrides, of which GaN is the archetype96 and the current mainstay of so many light-emitting diodes (LEDs); through InP and related compounds97 widely used for the fabrication of optoelectronic devices; operating at longer wavelengths and high-frequency devices to compounds such as InAsxSb1-x, which may offer the limit in III-V device performance in terms of speed, long wavelength for optoelectronics, and quantum effects related to low effective mass.94 Indeed, it was the availability of high-quality InP that triggered the rapid development of optical telecommunications during the 1980s and early 1990s. Signs of the huge research investment that continues to be made in the production, processing, and application of such compounds are to be found in a wealth of books, reviews, research reports, and original papers.96-98 Numerous indium compounds, known or hypothetical, with relevance to halide transport and chemical vapor deposition processes have been evaluated by their structural, electronic, and thermochemical properties on the basis of ab initio and statistical thermodynamic methods.99 Other speciality materials include indium oxide (In2O3) and its tin- or zinc-doped variants, widely used in ultrasensitive gas detectors, as electrically conductive films that are transparent to visible light while reflecting IR radiation, and in solar cells and optoelectronic devices.25,93 In addition, indium metal, indium mono- and trihalides, and other compounds are being exploited increasingly as reagents or mediators in organic and organometallic synthe-


Pardoe and Downs

Table 1. Indium(0) Derivatives Identified in Matrix-Isolation Experimentsa carbonyls

other complexes

In‚CO (89) In(CO)2 (89, 110)b In‚InCO (89) In(CO)2‚In (89)

In‚N2 (115) In‚NH3 (116) In(PH3)n (n ) 1 or 2) (117) In‚OH2 (119) In(η2-C2H2) (122) In(η2-C2H4)n (n ) 1-3) (122)

naked atoms or clusters In (86, 127) In2 (25,91,131)

a Identified and characterized primarily on the basis of their IR or electronic spectra or both, supported by quantum chemical calculations. References are given in parentheses. b Characterized in addition by its EPR spectrum.

sis,25,100,101 for example, in Barbier-type102 and FriedelCrafts103 reactions, C-C bond formation,104 transmetallation, and reduction.105 Of particular note is the capacity of In mediation to bring about a variety of reactions in aqueous media,106,107 thereby making them friendly to the environment. In the same vein, chloroindate(III) ionic liquids are found to be versatile reaction media for Friedel-Crafts acylation reactions; the system is catalytic and totally recyclable, using an aqueous workup, with no leaching of the metal into the product phase.103 There is some justice therefore in the recent claim that the full synthetic potential of indium in organic reactions has yet to be realized.106 This, then, is the context in which the present review aims to survey the known chemistry of indium(0), indium(I), and indium(II). It does not treat the negative oxidation states, which feature in intermetallic compounds, for example, Zintl phases, in which intriguing aggregates such as [In11]7- and [In28]n- may be perceived but usually as part of a more extended network; this subject has been amply covered for the p-block metals at large in several recent reviews.4,25-30 On the other hand, particular emphasis will be given here to the results of recent research, including the few indium clusters made so far, and to comparisons with the behavior of the lighter congeners of group 13. Subvalent boron, aluminum, and gallium compounds have been accessed mainly through their monohalides. Although indium monohalides have the advantage of ready availability and have indeed been used to prepare a number of indium(I) compounds, problems of solubility and of facile redox reactions have tended so far to hamper more ambitious exploration of this aspect of indium chemistry.

2. Indium(0) The limited experience of normal conditions teaches us that discrete indium(0) derivatives In‚nL are no more likely to survive decomposition to the bulk metal and the free ligand L than are analogous derivatives of other main group metals. That such species are formed has been demonstrated, however, notably by matrix-isolation experiments;84,86 Table 1 gives a listing. Some of them are surely noteworthy not only for their potential involvement in vapor transport of the metal but also in relation to the reactivity of the metal atoms.

2.1. Indium Carbonyls Main group metals, unlike d-block metals, do not form derivatives with neutral molecules such as CO, PF3, or C6H6 that are lastingly stable under ambient conditions. The first sign that the group 13 metal atoms are in fact capable of

binding CO came in 1972 with the report108 that the solid matrix formed by co-condensing Al atoms with an excess of krypton doped with CO at about 20 K displays new bands in the ν(CO) region of the IR spectrum that are red-shifted by ca. 150 cm-1 with respect to the corresponding band of free CO. Subsequent studies involving different matrices and EPR as well as infrared (IR) methods of detection109 have established beyond doubt that the bands belong to the angular Al(CO)2 molecule formed spontaneously from the metal atoms and CO; they have also led to the identification of AlCO and the bimetallic species Al2CO and Al2(CO)4. Similar matrix experiments with thermally evaporated Ga or In in place of Al atoms have revealed that the following molecules are formed on deposition: MCO, M‚MCO, M(CO)2, Ga(µ-CO)Ga, and In(CO)2‚In (M ) Ga or In), the proportions of which vary markedly with the concentrations of metal and CO.88,89 At all but the lowest concentrations of CO, In(CO)2 proves to be the major product in the indium experiments. Trapped in a solid argon matrix, the molecule is characterized by two intense IR absorptions at 2029 and 1980 cm-1 associated with the ν(CO) modes;89 it has also been identified independently by its EPR spectrum.110 It resembles Al(CO)2 and Ga(CO)2 in having an angular structure 9; on the evidence of the IR spectrum and DFT

calculations, this is remarkable for having a tight C-In-C angle (approaching 60°) and In-CtO arms that are bent outward in such a way as to suggest that the two carbon atoms are drawn toward each other. The geometry may reflect simply the achievement of optimum overlap between the vacant np valence orbital of the metal co-incident with the twofold axis of the molecule and the π-orbitals of the CO ligands. There are, however, alternative explanations, namely, (i) a repulsive interaction between the metal ns2 electrons and the 5σ donor pair on CO, (ii) a C‚‚‚C interaction between the two CO groups made admissible to the bonding scheme, say, through an OdCdCdO contribution to the ground-state wave function, or (iii) Coulombic repulsion between the terminal O atoms.88,89 With mean ν(CO) wavenumbers of 1946, 1967, and 2007 cm-1 for Al(CO)2, Ga(CO)2, and In(CO)2, respectively, it appears that the indium compound is the least strongly bound, a view endorsed by calculated binding energies of 176, 125, and 85 kJ mol-1 for the process 1 with M ) Al, Ga, and In in

M (2P) + 2CO (1Σ) f M(CO)2 (2A1)

(1)

that order.88 The mean binding energy per CO, at about 40 kJ mol-1 for In(CO)2 (and InCO), may be only half that for Al(CO)2 and less than a third that for Ni(CO)4 (148 kJ mol-1 111) and offers a poor return for the atomization energy of the metal (∆H°f,298[In(g)] ) 243 kJ mol-1 25), but it still represents a relatively strong interaction for a species that might otherwise be dismissed as an ephemeral van der Waals complex. At the very least, the In-CO bond is energetically comparable with a strong hydrogen bond such as that between carboxylic acid molecules.112 The known photolability of transition-metal carbonyls84 has prompted an investigation of the action of UV light on the matrix-isolated gallium and indium carbonyls.89 Whereas


Ga(µ-CO)Ga then takes up an additional CO molecule to produce the symmetrical dibridged Ga(µ-CO)2Ga molecule, photoactivation of In(CO)2 results in metal-to-ligand electron transfer with the coupling of two CO molecules to produce the ion-pair In+[OCCO]•-. Calculations show an increased electron density between the two carbon atoms in the first electronically excited-state of In(CO)2 and so hint at the mechanism of this photoisomerization. A similar but less selective reaction occurs on photolysis of CO-doped matrices containing Na atoms.113 Not only does the reaction with CO provide therefore a simple example of In mediation of C-C bond formation, it also highlights a feature that is likely to be important in electron transfer to organic substrates at large, namely, that In has a first ionization potential similar to that of an alkali metal (and lower than that of any other group 13 element).

2.2. Other Adducts of Indium Atoms Matrix experiments have also been carried out with In atoms isolated in argon matrices doped separately with H2,114 N2,115 NH3,116 PH3,117 NO,118 H2O,119 O2,120 CH4,121 C2H2,122 and C2H4.122 In the cases of H2 and CH4, it has not been possible to detect in the IR spectra of the deposits any new bands that can be ascribed to M‚H2 or M‚CH4 adducts, although the presence of such contact pairs is implicit in the photochemical changes subsequently displayed.114,121 Otherwise, the matrices reveal through their IR and electronic spectra evidence of interaction between the metal atom in its ground electronic state and the guest molecule L. The dominant product formed under these conditions is a 1:1 adduct, In‚L, although several of the systems show signs that the metal atom is able to bind more than one L molecule. Depending on the donor and acceptor properties of L, there is a wide variation in the strength of the binding in In‚L and in the degree and even direction of charge transfer. At one extreme, N2 forms only a loosely bound In‚N2 pair characterized by a ν(NN) vibration that is red-shifted by only about 15 cm-1 with respect to free N2;115 at the other extreme, O2 forms strongly bound In(η2-O2), the ν(OO) vibration of which is red-shifted by about 470 cm-1 with respect to free O2.120 In the second case, electron transfer from the metal to the O2 has proceeded virtually to completion, so that the product may reasonably be formulated as In+O2•-. DFT calculations yield the following binding energies (in kJ mol-1) for selected adducts:88 In‚N2, 7; In‚PH3, 17; and In‚NH3, 48. A similar pattern is found for the corresponding adducts of Al and Ga atoms, but for a given partner, the binding energy generally decreases appreciably in the order Al > Ga > In. With respect to the metal atom, molecules such as NH3 and H2O, which lack low-lying vacant orbitals, actually function as the donor partner, with the counter-intuitive result that charge transfer proceeds, albeit to a very limited degree, from ligand to metal. In the adduct In‚PH3, however, the transfer is in the opposite sense, producing a slight positive charge on the metal atom. A few weakly bound adducts of neutral In atoms have been studied in the gas phase through the media of fluorescence excitation and emission and two-color photoionization spectroscopies. These include the noble gas species In‚Ar, In‚Kr, and In‚Xe, for which the estimated dissociation energy of the X 2Π3/2 ground state increases, as expected, in that order.123 A more detailed study of In‚Ar finds that the excited B 2Σ1/2 state has an equilibrium bond length re ) 3.2778(9) Å and a dissociation energy, De, estimated to be


about 319 cm-1 (3.8 kJ mol-1), as compared with re ) 3.77(1) Å and De ) 192 cm-1 (2.3 kJ mol-1) for the X 2Π3/2 ground state.124 By contrast, the N2 complex In‚N2 appears to be more strongly bound in its ground electronic state (D0 ) ca. 1519 cm-1 or 18 kJ mol-1, cf. the theoretical estimate)88 than in the two excited states that have been identified.125 It is, however, appreciably but predictably, less strongly bound than the ion-molecule In+‚N2 for which D0 has been estimated to be 4817 cm-1 (58 kJ mol-1). The D0 estimated experimentally for the neutral complex In‚N2 conceals a significantly greater intrinsic binding energy (given by the fragmentation energy needed to form groundstate In atoms and unrelaxed N2 molecules) since much of this intrinsic energy is expended in extending slightly the NtN bond.126 Adduct formation by the metal atom in its ground electronic state is normally opposed by little or no activation barrier. However, the degree of activation is small for all but the most oxidizing of reagent molecules (e.g., NO and O2),118,120 and further reaction, even involving electron transfer but particularly where a bond has to be cleaved, requires a major input of energy. Under matrix conditions, this is most readily brought about by UV photolysis, which promotes the In atom in the adduct into its 2S or 2P excited electronic state (thereby injecting ca. 290 or 390 kJ mol-1 of extra energy into the system).86,127 Detailed quantum chemical studies of related systems suggest that such excitation strengthens the binding of the metal atom to the substrate molecule.86 In the particular case where the molecule is H2 or CH4, insertion of the metal atom into a H-H or C-H bond is then met with a greatly reduced, even zero, activation barrier. With indium, the first product is a genuine divalent indium compound, for example, HInH,114 CH3InH,121 HInNH2,116 HInPH2,117 or HInOH.119 Some matrix studies involve laser ablation as the means of generating the metal atoms, including a significant fraction in excited electronic states; the high energy of these atoms is apt to make them unselective in the reaction paths they follow. It is surprising, for example, that co-condensation of laser-ablated In atoms with N2 gives, among other products, the linear (4Πu) NInN molecule, as well as InN and InN3, but the rupture of the NtN bond is almost certainly brought about not by the In atoms but by the vacuum-UV radiation emanating from the focused laser plume at the metal target.128 Likewise, the reactions of laserablated In atoms with HCN, [CN]2, or CH3CN in affording the molecules InCN and InNC appear to bypass both In0 and InII derivatives.129

2.3. Naked Metal Clusters Despite a major energetic advantage over the bulk metals, Ga and In atoms prove not to be particularly reactive, irrespective of the thermodynamic balance of the prospective changes. Only on photolysis can Ga atoms be induced to react under matrix conditions with H-Y molecules to form the corresponding GaII product HGaY for Y ) H,114 NH2,116 PH2,117 OH,119 CH3,121 and SiH3,130 although the reaction with SnH4 appears to be spontaneous.130 This makes it all the more remarkable that the Ga2 dimer under similar conditions should react spontaneously not only with H2 (which see),91 but also with SiH4 [probably to form HGa(µ-SiH3)Ga], as well as SnH4 [probably to form the cluster Ga2(µ-H)4Sn].130 The In2 dimer is also known. On the limited spectroscopic evidence available for the gaseous molecule,131 it is likely


Figure 4. Thermally and photolytically activated reactions occurring in Ar matrices (a) between Ga2 and H2 and (b) between In2 and H2. Reprinted with permission from ref 91b. Copyright 2002 American Chemical Society.

to resemble Ga290 in having a 3Πu ground electronic state; the best experimental estimate of the dissociation energy gives D°0 ) 74.4 ( 5.7 kJ mol-1 132 (cf. 110.8 ( 4.9 kJ mol-1 for Ga2).132 Quantum chemical calculations at various levels of sophistication have been carried out not only on In2 but also on In3.99,132 Although the metal vapor at low pressure contains only very low concentrations of In2, trapping of the vapor in an excess of argon results typically in the presence of a small but significant fraction of the dimer. Unlike Ga2, however, matrix-isolated In2 gives no sign of a spontaneous reaction with H2.91 Such a reaction can be induced only by irradiation of the matrix with UV light having λ ) ca. 365 nm to form the dimeric indium(I) hydride In(µ-H)2In, with properties closely resembling those of the corresponding gallium compound.91 Both these molecules prove to be photolabile under visible light (λ > 450 nm). Exposure of the gallium compound to green light (λ ) ca. 546 nm) results in its conversion to two distinct isomers, namely, HGaGaH and GaGaH2, with the geometries illustrated in Figure 4a. All three isomers can be interconverted by an appropriate choice of wavelength for the photolyzing radiation or by annealing. By contrast, irradiation of the indium compound with light having λ > 450 nm gives rise to only a single isomer, HInInH, together with InH, in a change that can be reversed, as indicated in Figure 4b, by photolysis at λ ) ca. 365 or >700 nm. The experimental identification of the different isomers of Ga2H2 and In2H2 must be seen as a major triumph for the theoretical studies that foreshadowed group 13 subhydrides of this type.133 Yet experiment does more than vindicate theory, for “nothing ever becomes real till it is experienced”. On the other hand, the contrasting reactivities of Ga2 and In2 in these reactions pose a teasing problem. If there is as yet no obvious explanation, it must be appreciated that the physical and chemical characterization of these simplest of clusters is still only in its infancy. About larger indium clusters even less is known. Laser evaporation or ion bombardment (sputtering) of the metal or a compound of the metal can deliver to the gas phase not only atoms and dimers but also larger clusters in either neutral or charged states.4,134 These species have been investigated experimentally by different types of mass

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spectrometry (e.g., secondary ion mass spectrometry (SIMS) and Fourier transform (FT) ion cyclotron mass spectrometry), photoionization spectroscopy, or even calorimetric measurements, and theoretically at levels ranging from simple shell models to more sophisticated DFT methods. So it is with the group 13 metals. Thus, aluminum clusters134a and, to a lesser degree, gallium clusters134b have excited considerable interest in the general search for a better understanding of how the transition is made from the atomic/molecular state to the bulk metal. How the physical and chemical properties of the clusters vary as a function of size has therefore been a primary focus of enquiry, with the constantly teasing issue of just when a cluster can be justifiably described as a “metal”. Intriguingly, DFT calculations for aluminum clusters, Aln, indicate that icosahedral packing is favored only for n ) 13, whereas decahedral packing is most stable with n near 55, and fcc packing is energetically preferred for n > 80.134c Experimental studies have also turned to the primary reaction steps involving the relatively stable Al13- cluster and HCl, to provide what may be regarded as “snapshots” of the dissolution of the base metal in an acidic medium.134d Although indium has featured in a number of experiments designed to determine the relative yields and properties of neutral and charged Inn clusters,134e reliable quantum chemical calculations are altogether more challenging than those for the lighter metals. Neutral and charged clusters with n up to 200 have indeed been produced by bombarding a pure indium surface with 15-keV Xe+ ions,134f but even the yields depend only partially, and to an indeterminate extent, on their intrinsic thermodynamic properties. For the present, then, it may be “a capital mistake to theorize before you have all the evidence”, but individual indium clusters composed of more than two metal atoms must rely mainly on the powers of precedent and intelligent guesswork for characterization of their structural and thermodynamic properties.

3. Indium(I) 3.1. General Characteristics and Subdivisions If compounds of indium(0) may seem arcane, those of indium(I) are relatively prosaic. For no group 13 element apart from thallium is the +1 oxidation state better defined or represented by more compounds lastingly stable under ambient conditions than for indium.25 Systematic entry to indium(I) chemistry is achieved mainly by way of a monohalide, namely, InCl, InBr, or InI, or a cyclopentadienide derivative such as (η5-C5H5)In. The relatively high energy of the 5s2 electrons, which remain more or less localized on the metal atom, makes such compounds susceptible to oxidation, for example, by H+(aq), or, in the absence of an oxidizing agent, to disproportionation to indium metal and either indium(III) or indium-indiumbonded indium(II) species. Evidence now abounds40,41,135-137 to show that oxidation of indium(I) is initiated in the preferred route not by a 2e- but by a 1e- pathway yielding metastable indium(II), which is then oxidized much more rapidly. The reaction is most easily brought about by an inner-sphere mechanism, which introduces a marked dependence on the nature of the ligands and their capacity to bridge and provide electronic communication between the two redox centers. By contrast, direct single-step outer-sphere transactions of two electrons appear to be unusually sluggish under normal circumstances (as a result of the characteristically high Franck-Condon barriers involved), although



Table 2. Monomeric Molecular Indium(I) Compoundsa gas phase at elevated temperature InH InF InCl InBr InI (η5-C5H5)In (η5-C5H4Me)In (η5-C5Me5)In InCN InNC In2O In2Se In2Te In2MoO4

E,IR E,MW,PE E,MW,GED,PE E,MW,PE E,MW,GED,PE GED GED GED MW MW GED GED GED GED

(81, 141) (131, 142-144) (131, 142-146) (131, 142, 144, 147) (131, 142, 145, 148, 149) (154) (155) (66) (150) (150) (151) (152) (152) (153)

isolated in a solid inert matrix at low temperature InH InCl InCN InCH3 InC2O2 InNH2 InNO InNC In(η2-O2) InOH InO3 In2O GaOIn In2Sc InSHc

IR IR IR IR IR IR IR IR IR IR IR IR IR X-ray X-ray

(114) (159) (129) (121) (89) (116) (118) (129) (120) (119) (120) (120,156) (156) (160) (160)

molecular solid at ambient temperatureb In(C6H3-2,6-Trip2) (C6F5)3B‚In(C6H3-2,6-Dipp2) (C6F5)3B‚In(C6H3-2,6-Trip2) (η5-C5Me4PPh2)In HB(3-tBuPz)3In HB(3-PhPz)3In HB(3,5-tBu2Pz)3In HB[3,5-(CF3)2Pz]3In (η5-P2C3tBu3)In (η5-P3C2tBu2)In In[(NDippCMe)2CH] In[{NDippC(CF3)}2CH] In[(NDipp)2CNCy2] In(µ-OtBu)3Sn

(71) (161) (161) (163) (172) (175) (172) (174) (170,171) (170,171) (46) (177a) (178b) (176)

a Methods of characterization: E ) electronic spectroscopy; IR ) IR spectroscopy; MW ) microwave spectroscopy; PE ) photoelectron spectroscopy; GED ) gas electron diffraction. References are given in parentheses. b All characterized by single-crystal X-ray diffraction. Trip ) -C6H2-2,4,6-iPr3; Dipp ) -C6H3-2,6-iPr2; Pz ) pyrazolyl; Cy ) cyclohexyl. c Molecules trapped in fully In-exchanged zeolite A that has been exposed to H2S at elevated temperatures; measurements made at 294 K.

mediation through oxygen atom transfer from some hypervalent transition-metal species, for example, Cr(VI) and Cr(IV),138 may facilitate this mechanism. The disproportionation of indium(I) in aqueous solution proves to be complicated.135 While the predominant reaction in acid solution involves initially oxidation of InI to InII by H+ leading to an induction period, the formation of colloidal indium metal leads to heterogeneous disproportionation. Hence the following reaction scheme 2 has been proposed, where step a is rate-limiting and steps b and c are in competition:

(a) In(I) + H+ f In(II) + 1/2H2 (b) 2In(II) f In(I) + In(III)

(2)

(c) In(I) + In(II) f In(0) + In(III) Irrespective of their precise mechanism, the redox reactions to which a monomeric indium(I) species, InX, is subject are necessarily associative and dependent on the nature of both X and the environment. Long-term survival of InX can then be achieved in one of three ways: (a) by formation in the gas phase at low pressure, (b) by trapping in a solid inert matrix molecules formed either in the vapor or in situ, and (c) by the choice of a sufficiently bulky and non-oxidizing substituent X. Otherwise the InX molecules, left to their own devices, aggregate with the formation of either discrete oligomers or extended polymeric arrays. This reflects the polar character of the InX unit, but in contrast to the structural patterns displayed by analogous alkali metal compounds, aggregation is liable to be influenced, at least in part, by weak metal-metal attractions,139 which may in themselves be the prelude to disproportionation.140 In the following account, we will subdivide compounds containing indium exclusiVely in the +1 oxidation state into two categories, namely, (i) molecular compounds, which may be monomers, discrete oligomers, or loosely bound polymers, and (ii) compounds containing more or less well-defined In+ cations with strongly bound extended structures in the solid state. Compounds including InI as well as InII or InIII will be treated, together with those that defy the assignment of an integral oxidation state, in section 5.

3.2. Molecular Indium(I) Compounds 3.2.1. Monomers Table 2 lists according to phase indium(I) compounds that have been identified as more or less discrete monomeric molecules. These are the predominant species in the vapors of indium(I) compounds at low pressures and elevated temperatures. So we learn from the electronic, vibrational, microwave, or mass spectra or the electron diffraction patterns of the vapors. Some compounds, such as InCl, InBr, InI, or (η5-C5R5)In (R ) H or Me), vaporize on heating without decomposition or disproportionation; at sufficiently high temperatures, the entropy advantage drives even robust InIII molecules such as In2O3 and InCl3 to decompose, at least partially, to the corresponding InI compound and elemental nonmetal. Other InI compounds, such as InH, InF, InCN, or InNC, require high-energy reactions for their formation.141-153 For example, InH is generated by the reaction of the molten metal with H2 at 1000-1200 °C prior to characterization by its high-resolution IR spectrum measured either in emission or in absorption.81,141 In a similar vein, InF is formed by heating together the metal and InF3,144 while the reaction of laser-ablated In vapor with [CN]2 or CH3CN in an Ar carrier gas affords the two isomers InCN and InNC, whose structures and vibrational/rotational properties have been fixed by their microwave spectra.150 The electron diffraction patterns of the derivatives CpIn, where Cp ) C5H5,154 C5H4Me,155 and C5Me5,66 serve to confirm the η5-coordination of the cyclopentadienyl ring. Rather less certain is the angular structure deduced on similar grounds for In2O;151 in that more recent matrix and theoretical studies favor a linear structure,120,156 the apparent angularity may well signify failure to model adequately a skeletal bending vibration of low wavenumber and large amplitude. The properties determined for the molecules InH, InF, InCl, InBr, and InI are compared in Table 3 with those of the analogous rubidium and silver compounds.81,111,131,157 The microwave spectra of these MX species allow the equilibrium bond lengths to be determined with prodigious accuracy (often limited only by the uncertainties in the values of the relevant fundamental physical constants). These lengths follow a consistent pattern, namely, RbX > InX > AgX. The magnitudes of the vibrational parameters ωe and ωexe


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Table 3. Comparison of the Properties of the Diatomic Hydride and Halide Molecules Formed by Rubidium, Silver, and Indium r e, Å

ω e, cm-1

ωexe, cm-1

D°298, kJ mol-1

RbH 107 AgH

2.366808 1.617798

937.1046 1759.7436

14.278 33.977

167 215

115

InH

1.835971

1475.4183

25.143

243

85

RbF AgF 115 InF

2.270333 1.983179 1.985400

376 513.447 535.00

1.9 2.593 2.62

494 354 509

111, 131 111, 131 111, 141-143

l2.786736 l2.280792 l2.402024

228 343.49 317.389

0.92 1.17 1.032

428 314 439

111, 131 111, 131 111, 141, 142, 144

Rb79Br Ag79Br 115 79 In Br

2.944744 2.39311 2.543179

169.46 249.57 222.93

0.463 0.63 0.52

381 293 414

111, 131 111, 131 111, 141, 142, 146

85

3.176879 2.544621 2.753618

138.51 206.50 177.08

0.335 0.46 0.34

319 234 331

111, 131 111, 131 111, 141, 146

molecule 85

107

85

Rb35Cl Ag35Cl 115 35 In Cl 107

85

107

RbI AgI 115 InI 107

ref 81, 111 81, 111, 157 81, 111, 140

follow the reverse order, with the exception of the fluorides for which the sequence is InF > AgF > RbF. The highly Coulombic interaction in RbX yields increasingly to charge transfer and covalence as Rb is replaced by In and then by Ag. The dissociation energies, D°298, may be known much less reliably, but the values are interesting for the different picture they paint. In all cases, it is now indium that forms the strongest bond and silver generally the weakest. Repulsive interactions involving the 4d10 shell of silver must play a part in weakening the Ag-X bonds, but halide molecules like InF and InCl must draw some of their high dissociation energies from π-bonding. In simple valence terms, they are isoelectronic with CO. Ab initio MP2 and CCSD(T) calculations on GaCl and InCl158 confirm this analogy, revealing the frontier orbitals to be as portrayed in Figure 5. Thus, the HOMO housing the ns2 electron pair and confined largely to the metal has σ symmetry.142,158 Interactions between the valence p orbitals then give relatively strongly bound σ and π orbitals localized mainly on the halogen and high-energy antibonding orbitals of mainly metal np character (n ) 4 or 5) that include the π-symmetry LUMO. In one form or another, these features are characteristic of all MX-type molecules formed by the group 13 metals (M) with either a halogen or an organic group (X),56,73,75,77,78 although the relative energies of the HOMO and LUMO vary from metal to metal and substituent to substituent. Molecules such as InCl and In2O may also be detected and interrogated, usually by their IR spectra, by cocondensing the vapor with an excess of an inert gas. Alternatively, the indium(I) compounds may be formed in situ by UV photolysis of a matrix-isolated indium(II) compound of the type HInX, itself formed by photoactivation of the metal atoms in the presence of HX molecules (X ) H,114 CH3,121 NH2,116 or OH119). Decomposition of HInX then proceeds in accordance with eq 3. Appropriate quantum

chemical calculations typically play a part not only in

Figure 5. (a) Frontier orbitals of InCl (1Σ) and (b) potential energy curves for the molecule in its 1Σ ground electronic state and 3Π excited state (adapted from ref 158).

endorsing the identification of a given InI compound but also in elaborating its structural, spectroscopic, and thermodynamic properties. In some cases, the matrix may act as host to reactions of the compound. For example, IR measurements have shown that although InCl resembles AlCl in being relatively unreactive in its 1Σ ground electronic state, excitation to its 3Π state results in insertion reactions with HX molecules that lead to the InIII compounds HXInCl, where X ) H,159 Cl,159 or OH,158 as in eq 4. Because of its

symmetry, the excited-state of InCl can establish attractive interactions with the σ* orbital of HX. Reaction 4 is calculated to be endothermic, unlike the corresponding reactions with AlCl and GaCl but reflecting the weaker bonds formed by indium. However, a substantial activation barrier of ca. 300 kJ mol-1 opposes the reversal of the reaction.158 Insufficient mobilization can be achieved by annealing a noble gas matrix to promote significant aggregation of monomeric guest molecules, and in those cases where the IR spectra suggest the presence of dimers or higher oligomers, for example, In2H2,91 the molecule or its precursor must either be present in the vapor prior to condensation or be formed while the vapor and matrix gas are being quenched. Matrices of a rather different (and more reactive) kind are provided at ambient temperatures by zeolites. The sodium ions of zeolite A may, for example, be replaced at least formally by indium, which is then present not just as In+ but in various oxidation states (0, +1, +2, and +3) and degrees of aggregation. Exposure of a single crystal of such a zeolite to H2S at temperatures in the range 298-573 K results in coloration, and X-ray analysis indicates the presence of numerous products, including In2S and InSH, held in the cavities of the host.160



Figure 6. Structures of some molecular mononuclear indium(I) compounds in the crystalline state: 10, In(η1-C6H3-2,6-Trip2) (Reprinted with permission from ref 71. Copyright 1998 American Chemical Society); 11, In(η5-P2C3tBu3) (Reprinted with permission from ref 170. Copyright 2000 Royal Society of Chemistry); 12, HB(3,5-tBu2Pz)3In (Reprinted with permission from ref 172. Copyright 1996 American Chemical Society); 13, In[(NDippCMe)2CH] (Reprinted with permission from ref 46. Copyright 2004 Royal Society of Chemistry); 14, In[(NDipp)2CtBu] in In[(NDipp)2CtBu]‚Dipp(H)NC(tBu)NDipp (Reprinted with permission from ref 178a. Copyright 2005 Royal Society of Chemistry).

Only through coordination to a ligand that is exceptionally demanding of space around the metal can monomeric indium(I) compounds be made to survive in the condensed phases at ambient temperatures. Perhaps the most notable example is the unusually encumbering o-terphenyl ligand -C6H3-2,6-Trip2, where Trip ) C6H2-2,4,6-iPr3; hence the compound InC6H3-2,6-Trip2 has been prepared and shown to form crystals composed of well-separated monomers 10 (see Figure 6), which are unique in the one-coordination of the metal.71 Even relaxing the bulk of the ligand seemingly quite slightly in the change from -C6H3-2,6-Trip2 to -C6H32,6-Dipp2, where Dipp ) C6H3-2,6-iPr2, gives not a monomer but a “dimetallene” dimer [InC6H3-2,6-Dipp2]2 with an InIn bond.161 The Lewis base character of the organoindium(I) monomer is brought out by the formation of the monomeric complex (C6F5)3B‚InC6H3-2,6-Ar2 (Ar ) Trip or Dipp). This too is unusual in featuring more or less linear, 2-fold coordination of the metal. All the alkylindium compounds of the type InC(SiR2R′)3 (R, R′ ) Me, Et, iPr, nBu, or Ph) for which crystal structures are known are tetrameric, but the extra size of the C(SiMe2iPr)3 and C(SiMe2Ph)3 groups causes the relevant indium(I) derivatives to be monomeric in benzene solution at ambient temperatures.162 Otherwise the necessary steric bulk is provided by multidentate ligands. Some of the best known organoindium(I) compounds are the cyclopentadienyl derivatives,144,155,163-169 but even here the “bite” of the pentahapto ligand is generally insufficient to prevent aggregation in the crystalline state. Whereas the pentamethylcyclopentadienyl compound is a loosely bound hexamer,66 however, increasing the bulk of the ligand by substituting Ph2P for one of the CH3 groups does favor a monomeric

molecular crystal structure in which the metal atom is simply η5-coordinated to the cyclopentadienyl ring.163 The presence of tBu substituents makes the related phosphacyclopentadienyl ligands P3C2tBu2- and P2C3tBu3- no less spatially demanding, and these too form monomeric crystalline InI derivatives (e.g., 11 in Figure 6).170,171 Somewhat less sensitive to the choice of substituent in screening InI centers are tris(pyrazolyl)borate anions. Irrespective of whether substitution occurs at both the 3 and 5 positions of the pyrazolyl ring and of whether the substituent is tBu,172,173 CF3,174 or Ph,175 the indium(I) compounds crystallize as monomeric structures. For example, HB(3,5-tBu2Pz)3In (Pz ) pyrazolyl) adopts the structure 12 (Figure 6), interaction between the tBu groups in the 5-positions causing the ligands to assume a highly twisted configuration. Similar structural principles operate for the tridentate Sn(OtBu)3 ligand in the otherwise quite different compound In(µ-OtBu)3Sn.176 β-Diketiminato anions of the type (ArNCR)2CH- may be only bidentate, but with sufficiently bulky aryl groups, such as Ar ) Dipp, and R ) Me46 or CF3,177a they are capable of forming monomeric InI compounds with 2-fold coordination of the metal and an N-In-N angle close to 80°, as represented, for example, by the structure 13 (Figure 6) of In[(NDippCMe)2CH].46 Synthetic strategies appropriate to the preparation of such six-membered InI heterocycles have been reviewed;177b their reactivity testifies to their carbenelike character.177b,c The anion [(NDipp)2CtBu]- also forms a monomeric InI derivative isolated as In[(NDipp)2CtBu]‚Dipp(H)C(tBu)NDipp in which the metal center is N,N′,N′′chelated by the amidinato ligand in an η3-arene fashion, 14.178a A different mode of coordination involving N,N′-


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Figure 7. Structures of some molecular, loosely associated indium(I) compounds in the crystalline state: 15, [In{N(C6H2-2,4,6-Me3)CMe}2CH]2 (Reprinted with permission from ref 179. Copyright 2005 Wiley-VCH); 16, unit cell of [In(η5-C5H5)]n (Reprinted with permission from ref 164. Copyright 1991 Springer-Verlag, Berlin, Heidelberg); 17, [In(η5-C5Me5)]6 (Reprinted with permission from ref 66. Copyright 1989 American Chemical Society); 18, [In{η5-C5(CH2Ph)5}]2 (Reprinted with permission from ref 164. Copyright 1991 Springer-Verlag, Berlin, Heidelberg); 19, [InC(SiMe3)3]4 (Reprinted with permission from ref 182b. Copyright 1995 Elsevier); 20, [In(η1-C6H3-2,6-Dipp2)]2 (Reprinted with permission from ref 161. Copyright 2002 American Chemical Society.).

chelation of M(I) (M ) Ga or In) is afforded by the guanidinate anion [(NDipp)2CNCy2]- (Cy ) cyclohexyl) to give monomeric four-membered heterocycles of the type M[η2-(NDipp)2CNCy2], analogous to similar N-heterocyclic carbenes and offering inviting prospects as σ-donor ligands.178b On the other hand, relaxing the steric demands of the aryl group by replacing Dipp in 13 by mesityl (C6H2-2,4,6-Me3) permits dimerization in the solid state through the formation of a direct In-In bond.179 The centrosymmetric dimer 15 (Figure 7) possesses approximate C2h symmetry with a mutual trans-bent orientation of the N-chelated ligands, so that each In atom has a pyramidal environment analogous to that of the Sn atoms in a homoleptic stannylene of the general formula R2SnSnR2. As befits the increased coordination number at the metal atom, the In-In distance [3.1967(4) Å] is significantly longer than that in [InC6H3-2,6Dipp2]2.161 No structure has yet been established for a simple amidoindium(I) compound. The bis(trimethylsilyl)amido derivative, InN(SiMe3)2, decomposes at ambient temperatures and is recognizable only by its spectroscopic properties and certain chemical reactions.52 On the evidence of its thallium analogue, which is made up of cyclic dimers in the solid state,180 it is unlikely to be monomeric. Increasing the size of the substituents does give a monomeric thallium(I) compound in TlN(Me)Ar, where Ar ) -C6H3-2,6-mesityl2,181 but no indium counterpart has yet been reported. Access to both monomeric and related oligomeric derivatives of indium(I) is usually gained by metathesis between an indium(I) halide and an alkali-metal derivative of the

relevant ligand (see Table 4). Somewhat exceptionally, the phosphacyclopentadienyl compounds have been prepared by co-condensation of indium metal vapor with tBuCtP at 77 K.170,171 Inasmuch as the oligomers are quite weakly bound, dissociation is commonly the first step in their reactions. In many ways, therefore, the chemistry of molecular InI species is influenced but little by their state of aggregation in the solid state or even in solution at ambient temperatures. Accordingly, it is appropriate first to review the structures favored by the oligomers in the condensed phases before surveying the chemical characteristics of molecular InI compounds at large.

3.2.2. Oligomers or Weakly Bound Polymers η5-Coordination of an indium(I) center by a cyclopentadienyl or similar ring yields solid compounds that run the gamut from the polymeric, through the oligomeric, to the monomeric. With the less hindered cyclopentadienyl ligands C5H5-, C5H4Me-, C5H4E(CH3)3- (E ) C or Si), and C5HMe4-, homopolymeric zigzag chains, for example, 16 in Figure 7, are the rule, with the metal atom sandwiched between adjacent rings and lying on the C5 axis of each of them. Where comparisons can be drawn, the metal-to-ring centroid distance is substantially longer than that in the gaseous monomer (cf. 2.71 vs 2.32 Å for InC5H5).154,164 The metal atoms are separated by distances ranging from 3.99 to 5.43 Å or more,164,165 which hint at an interaction that is at best only weak. With increasing bulk of the cyclopentadienyl unit, however, nonbonded repulsions between these ligands become the dominant influence, and polymerization



Table 4. Preparation and Characterization of Molecular Indium(I) Compounds compounda In(µ-H)2In, HInInH [InC(SiMe3)3]4 [InC(SiMe2Et)3]4 [InC(SiMe2iPr)3]4 [InC(SiMe2nBu)3]n [InC(SiMe2Ph)3]n [InC(SiMeEt2)3]n In(C6H3-2,6-Trip2) [In(C6H3-2,6-Dipp2)]2 (C6F5)3B‚In(C6H3-2,6-Trip2) (C6F5)3B‚In(C6H3-2,6-Dipp2) [NaIn(CH2SiMe3)2]n

[In(C5H5)]n [In(C5Me5)]6 [In(C5H4Me)]n [In(C5HMe4)]n [In(C5H4tBu)]n [In(C5H4SiMe3)]n [In{C5H3(SiMe3)2}]n [In{C5H2(SiMe3)3}]n [In(C5H4GeMe3)]n In(C5H4PPh2) [In(C5H3-1,3-Ph2]n [In{C5(CH2Ph)5}]n [In{C5(C6H4-4-Et)5}]n [In{C5(C6H4-4-COMe)5}]n [In{C5(C6H4-4-COC5H11)5}]n [(η6-toluene)In(µ-η5-C5Me5)In(η6-toluene)]+ [(C6F5)3B(µ-OH)B(C6F5)3][In(µ-η5-C5Me5)In]+[B(C6F5)4]In(P3C2tBu2) In(P2C3tBu3)

or Hydride Groups photolysis of an Ar matrix doped with In2 + H2

IR

91

η1-Coordinated Organic Groups InBr + LiC(SiMe3)3‚xTHF in toluene InBr + LiC(SiMe2Et)3‚xTHF in toluene InBr + LiC(SiMe2iPr)3‚xTHF in toluene InBr + LiC(SiMe2Bun)3‚xTHF in toluene InBr + LiC(SiMe2Ph)3‚xTHF in toluene InBr + LiC(SiMeEt2)3‚xTHF in toluene InCl + LiC6H3-2,6-Trip2‚OEt2 in THF InCl + LiC6H3-2,6-Dipp2 in toluene In(C6H3-2,6-Trip2) + B(C6F5)3 in toluene In(C6H3-2,6-Dipp2) + B(C6F5)3 in toluene In(CH2SiMe3)3 + NaH in hexane, benzene, or dimethoxyethane

X-ray, NMR, IR, UV/vis, MS X-ray, NMR, IR, UV/vis, MS X-ray, NMR, IR, UV/vis, MS NMR, IR, UV/vis, MS NMR, IR, UV/vis, MS NMR, IR, UV/vis, MS X-ray, NMR, UV/vis, MS X-ray, NMR, UV/vis X-ray, NMR X-ray, NMR NMR, IR

182 162 162 162 162 162 71 161 161 161 183

X-ray, GED, PE, NMR, IR, MS

155, 164

X-ray, GED, NMR, IR X-ray, GED, NMR, IR, MS X-ray, NMR, MS X-ray, NMR, IR, MS X-ray, NMR, IR, MS NMR, MS NMR, MS NMR, IR, MS X-ray, NMR, MS NMR, MS X-ray, NMR, IR, MS NMR, MS NMR, MS NMR, MS X-ray

66, 164 144, 164 165 164, 166 164 164 164 164 163 167 164 168 168 168 169

X-ray X-ray, NMR, MS, PE X-ray, NMR, MS, PE

69 170,171 170,171

NMR X-ray, NMR X-ray, NMR X-ray, NMR, MS X-ray, NMR, IR, MS X-ray, NMR

52 46 177a 178a 178b 179

X-ray X-ray, NMR, MS X-ray X-ray, NMR

172, 173 172 175 174

X-ray, NMR, IR, MS NMR, IR NMR, IR NMR, IR NMR, IR NMR, IR EPR, IR EPR, IR EPR, IR IR X-ray, NMR

190 185 185 185 185 185 186 186 186 186 176

NMR IR IR IR NMR, IR

187 188 188 188 189

NMR, IR

189

X-ray, NMR, IR

197

η5-Coordinated Cyclopentadienyl and Related Groups InX + LiC5H5 in Et2O; decomposition of In(C5H5)3; co-condensation of In + C5H6 InCl + LiC5Me5 in Et2O InCl + LiC5H4Me in Et2O InCl + LiC5HMe4 in Et2O InCl + LiC5H4tBu in Et2O InCl + LiC5H4SiMe3 in Et2O InCl + LiC5H3(SiMe3)2 in Et2O InCl + LiC5H2(SiMe3)3 in Et2O InCl + LiC5H4GeMe3 in Et2O InCl + TlC5H4PPh2 in Et2O InCl + NaC5H3-1,3-Ph2 in Et2O InCl + LiC5(CH2Ph)5 in Et2O InCl + NaC5(C6H4-4-Et)5} in Et2O InCl + NaC5(C6H4-4-COMe)5 in Et2O InCl + NaC5(C6H4-4-COC5H11)5 in Et2O InC5Me5 + B(C6F5)3 + H2O‚B(C6F5)3 in toluene InC5Me5 + [(toluene)H]+[B(C6F5)4]- in toluene co-condensation of In vapor + tBuCtP co-condensation of In vapor + tBuCtP

HB(3-tBuPz)3In HB(3,5-tBu2Pz)3In HB(3-PhPz)3In HB{3,5-(CF3)2Pz}3In [InOC6H2-2,4,6-(CF3)3]2 [InOC6H4-2-OH]n [InOC10H6-3-OH]n [In-2-OC12H8-2′-OH]n [Et3NH][In-1,2-O2C6H4] [Et3NH][In-2,3-O2C10H6] [In(TBSQ)‚phen]n [In(TBSQ)‚phen‚1.5Et2O]n [In(TBSQ)‚phen‚TBQ]n [Ph4P][In(TBSQ)Cl‚phen] In(µ-OtBu)3Sn

Oxygen-Coordinated Groups InC5H5 + HOC6H2-2,4,6-(CF3)3 in n-hexane In + C6H4-1,2-(OH)2 + Et4NClO4 in MeCNc In + C10H6-2,3-(OH)2 + Et4NClO4 in MeCNc In + C12H8-2,2′-(OH)2 + Et4NClO4 in MeCNc InOC6H4-2-OH + Et3N in MeCN InOC10H6-3-OH + Et3N in MeCN In + TBQ + phen in toluene In + TBQ + phen in toluene/Et2O In + TBQ + phen in toluene In(TBSQ)‚phen + Ph4PCl in MeCN InBr + M(µ-OtBu)3Sn (M ) Tl or Na) in toluene

[InSR]n (R ) Et or nBu) [InSCmH2mSH]n (m ) 2-6) [NEt3H][In(1,6-S2C6H12)] [NBu4][In(1,m-S2CmH2m)] (m ) 4 or 6) [InSeSi(SiMe3)3]n [InTeSi(SiMe3)3]n

In[(PhCH2)2GaCl2]

ref

µ2-Coordinated

Nitrogen-Coordinated Groups InCl + LiN(SiMe3)2 in THF InI + H(NDippCMe)2CH + KN(SiMe3)2 in THF InI + H{NDippC(CF3)}2CH + KN(SiMe3)2 in THF InCl + K[(NDipp)2CtBu] + Dipp(H)NC(tBu)NDipp in THF InCl + Li[(NDipp)2CNCy2] in toluene InI + H{N(C6H2-2,4,6-Me3)CMe}2CH + KN(SiMe3)2 in THF InCl + HB(3-tBuPz)3Tl in THF or benzene InCl + HB(3,5-tBuPz)3Na in benzene InI + [HB(3-PhPz)3]- in THF InCl + HB{3,5-(CF3)2Pz}3Ag‚THF in THF

[InN(SiMe3)2]n In[(NDippCMe)2CH] In[{NDippC(CF3)}2CH] In[(NDipp)2CtBu]‚Dipp(H)NC(tBu)NDipp In[(NDipp)2CNCy2] In2[{N(C6H2-2,4,6-Me3)CMe}2CH]2

characterizationb

how made η1-

Other Chalcogen-Coordinated Groups In + RSH + Et4NClO4 in MeCNc In + CmH2m(SH)2 + Et4NClO4 in MeCNc InSC6H12SH + NEt3 in MeCN InSCmH2mSH + [Bu4N]OH in MeCN InCl + LiSeSi(SiMe3)3‚THF in hexane or InC5H5 + HSeSi(SiMe3)3 in hexane InCl + LiTeSi(SiMe3)3‚2THF in hexane or InC5H5 + HTeSi(SiMe3)3 in hexane Halides InCl + (PhCH2)2GaCl in toluene. Consists of four-membered In2Cl2 rings weakly bound to a coordination polymer

a Trip ) -C H -2,4,6-iPr ; Dipp ) -C H -2,6-iPr ; Pz ) pyrazolyl; C H ) naphthalene; C H 6 2 3 6 3 2 10 8 12 10 ) biphenyl; TBSQ ) 3,4-di-tert-butyl-1,2o-benzosemiquinonate; TBQ ) 3,4-di-tert-butyl-1,2-o-benzoquinone; phen ) phenanthroline; Cy ) cyclohexyl. b X-ray ) single-crystal X-ray diffraction; MS ) mass spectrum; PE ) UV photoelectron spectrum. c Electrochemical oxidation.


gives way to oligomerization.66,164 Thus, InC5Me5 forms hexameric units each built on a roughly octahedral In6 skeleton, 17, with dimensions determined less by the strength of In-In bonding than by the strength of ligand-ligand repulsion.65,66 Replacing methyl by benzyl as the substituents of the C5 ring comes close to favoring a monomeric molecular unit;164 what the crystal structure of In[C5(CH2Ph)5] actually reveals are quasi-dimeric units, 18, in which the monomers are linked together through a tenuous In-In link measuring 3.631 Å (cf. 3.95 Å in [InC5Me5]6).66 Tetramers based on a tetrahedral In4 core are favored by η1-coordinated organic ligands of the type -C(SiMe2R)3 (R ) Me, Et, or iPr), for example, 19, in this case with significantly shorter In-In distances of 3.00-3.15 Å.162,182 The form of aggregation favored by the compound NaIn(CH2SiMe3)2 has yet to be established.183 As noted already, even the highly encumbering aromatic substituent -C6H32,6-Dipp2 lacks the bulk to prevent its InI derivative from forming dimers in the solid state.161 These take the form of centrosymmetric units with a trans-bent C-In-In-C skeleton, 20. The In-In distance of 2.9786 Å is similar to that in the tetramers; while being appreciably shorter than the corresponding distance in [In{C5(CH2Ph)5}]2, it still lies beyond the currently known range of 2.70-2.94 Å associated with what pass for In-In single bonds (which see). That the dimer is extensively dissociated in cyclohexane solution typifies the behavior of organoindium(I) compounds: no matter how the organic substituent is coordinated or what form of aggregation is assumed by the monomers in the solid, the intermolecular forces are persistently weak. Stronger metal-metal bonding might be expected to arise on reduction of the dimer to the hypothetical dianion [In(C6H3-2,6Dipp2)]22-, that is, a pseudo-alkyne and the indium analogoue of the known gallium species,49 thus bringing indium into the debate about homonuclear multiple bonding between the group 13 metals;57-64 to date, however, no such product has been identified. The compound [In(C6H3-2,6-Dipp2)]2 has been matched very recently184 by the isolation of a thallium(I) analogue, also featuring a planar, trans-bent structure. There is as yet, however, no known indium counterpart to the TlI trimer [Tl{C6H3-2,6-(C6H3-2,6-Me2)2}]3 centered on a three-membered Tl3 ring that results when the steric demands of the aryl ligand are eased somewhat. About the structures of other oligomeric indium(I) compounds in which the metal is coordinated to nitrogen, oxygen, or heavier group 16 atoms,185-189 definitive information is sparse. A rare example of authentication is provided when the metal is partnered with the sterically demanding 2,4,6tris(trifluoromethyl)phenoxide ligand.190 The crystal structure is composed of dimeric units 21 in which the two-coordinated

metal atoms are linked by phenoxide bridges, with the phenyl groups almost coplanar and orthogonal to the In2O2 ring. Notwithstanding the low coordination number of the metal, the In-O distance, averaging 2.320 Å, is substantially longer than those found in InIII compounds also having a cyclic In2O2 core unfettered by chelation (averaging 2.14 Å).191

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The possibility of stabilizing at normal temperatures a discrete cluster of a monohalide of the group 13 metal has been realized through the intercession of a suitable base in the case of aluminum. Thus, the adduct of AlX (X ) Br or I) with NEt3 or PEt3 (with X ) I) is a tetramer based on a square Al4 ring with the halogen and base substituents arranged in an alternating fashion above and below the ring.192 By contrast, attempts to produce similar clusters of gallium(I) halides have invariably met with at least partial disproportionation, with the isolation of mixed valence products such as Ga5Cl7‚5Et2O,193 Ga8I8(PEt3)4,194 or Ga10Br10(4-tert-butylpyridine)10195 containing nonequivalent metal atoms. On the very limited evidence at hand, indium follows the example of gallium, forming products such as [In5Br8(quinuclidine)4]- 196 (see section 5), and no discrete clusters made up exclusively of InI fragments have yet been reported. The closest approach to the stabilization of a discrete indium(I) halide cluster has been achieved with the compound In[(PhCH2)2GaCl2] formed by the reaction of InCl with (PhCH2)2GaCl;197 X-ray analysis shows that this consists of four-membered In2Cl2 rings connected by weak In‚‚‚Cl contacts to a coordination polymer.

3.2.3. Reactions of Molecular Indium(I) Compounds. Indium(I) compounds at large are characterized by the relatively weak acidity but significant basicity of the InI center and by their unsaturation. With regard to neutral and charged donor ligands, they present much less incentive for bonding than do corresponding indium(III) compounds; herein lie the seeds of disproportionation, usually to the metal and an InIII derivative, but sometimes to the metal and an InII derivative having an In-In bond (which see). As noted earlier, the lone pair of electrons originating in the 5s2 valence shell of the free In+ ion and concentrated largely on the metal is the prime source of basicity in a monomeric InI molecule such as (η5-C5Me5)In; coordination to an appropriate acidic reagent may then serve to stabilize the InI compound against redox reactions. However, the basic action of the compound is liable to lead ultimately to its oxidation, often with insertion of the InR moiety into a pre-existing bond of the reagent, and almost always to convert the metal to its +3 oxidation state. Against this background, selected reactions of the cyclopentadienylindium(I) compounds (η5-C5R5)In (R ) H or Me) and tris(trimethylsilyl)methylindium(I) are shown schematically in Figures 8 and 9, respectively. The bonding requirements and unusual bulk of the cyclopentadienyl ligand disfavor strong aggregation, and particularly clustering, of (η5-C5R5)In molecules, and the reactions appear invariably to be those expected of the monomeric molecules. The η1alkyl ligand C(SiMe3)3 differs from η5-C5R5 in both bonding and space requirements [giving rise to a significantly smaller cone angle for InC(SiMe3)3 than for In(C5R5)], with the result that clustering, while still involving only weak metal-metal bonding, has a stronger influence. Although dissociation to monomeric InC(SiMe3)3 is still a common prelude to reaction, the outcome of some reactions clearly suggests that the tetramer with its tetrahedral In4 cage is robust enough to act as a template for the formation of larger heteronuclear clusters. Such is the case, for example, under mild conditions of halogenation198,199 or oxidation by addition of a group 16 element.200-202 Otherwise organoindium(I) compounds can be seen to undergo the following types of change. (i) Metathesis with exchange of organic for other ligands. This is best illustrated by cyclopentadienylindium(I), which,



Figure 8. Some reactions of (η5-cyclopentadienyl)indium(I) compounds. Reagents: (i) BX3 (X ) F, Cl, Br or Me);39,164,210 (ii) CF3SO3H in toluene;203 (iii) [Et4N]X + HX in C6H6/EtOH solution (X ) Cl, Br or I);39,204 (iv) I2;39 (v) ROH [R ) C6H2-2,4,6-(CF3)3];39,190 (vi) CF3CdC(CF3)SS;39,205 (vii) B(C6F5)3 + H2O‚B(C6F5)3 in toluene;169 (viii) [(toluene)H]+[B(C6F5)4]- in toluene;69 (ix) Pd2(dvds)3 in n-hexane, where dvds ) 1,3-divinyl-1,1,3,3-tetramethyldisiloxane;73 (x) [Cp*RhCl2]2 in toluene;218 (xi) [Cp*RuCl]4 in n-hexane;219 (xii) heat in dry anisole containing polyvinylpyrolidone as stabilizer;221 (xiii) heat with PH3 in an H2 stream at 570-640 °C;222 (a) Cp ) C5H5 unless specified otherwise; in steps marked with b, Cp ) Cp* (C5Me5).

Figure 9. Some reactions of the alkylindium(I) compound [InR]4, where R ) C(SiMe3)3. Reagents: (i) C2Cl6 in toluene;198 (ii) Br2/AlBr3 in n-hexane;198 (iii) BrCH2CH2Br in toluene;198 (iv) I2/AlI3 in n-hexane;199 (v) oxygen donor (o-nitrosotoluene), elemental S, Se, or Te in n-hexane;200,201 (vi) propylene sulfide in n-hexane;202 (vii) M(cod)2 (cod ) cyclooctadiene; M ) Ni or Pt) in toluene;72,212 (viii) Mn2(CO)10 in n-hexane;213 (ix) Co2(CO)8 in n-hexane;214 (x) (cot)Fe(CO)3 (cot ) cyclooctatetraene) in n-hexane;215 similar reactions occur with Fe2(CO)9,216 Fe3(CO)12,216 and Cp2Ni(µ-CO)2217 with replacement of CO by InC(SiMe3)3; (xi) [Cp*RuCl]4 in n-hexane;219 (xii) ArCOCOAr (Ar ) Ph, 4-MeOC6H4 or 4-BrC6H4) in hexane;206 (a) R ) C(SiMe3)3; Cp ) η5-C5H5; Cp* ) η5-C5Me5.

by virtue of its solubility in organic solvents, has proved to be a useful precursor to other indium(I) compounds, for example, In+CF3SO3-,203 what are claimed to be salts of the [InX2]- anion (X ) Cl, Br, or I),39,204 and [In(µ-OAr)]2 [Ar ) -C6H2-2,4,6-(CF3)3].39,190 (ii) Oxidation. Relatively mild conditions are needed to bring about oxidation of organoindium(I) compounds, with InIII as the ultimate destination. Exposure to air or moisture

results in rapid attack; the reactions may be complicated, with disproportionation preceding oxidation under aqueous conditions, for example, but the net result is always oxidation of the metal to the +3 state.164 Such oxidation is also achieved by the halogens {to give, for example, (η1-C5H5)InI2},39 chalcogens {to give cluster compounds of the type [RInE]4 with a cubane-like In4E4 core [R ) C(SiMe3)3; E ) O, S, Se, or Te]},200,201 and reagents like 1,2-bis(trifluoro-


methyl)dithieten39,205 and benzil, [C6H5CO]2.206 The reactions typically involve insertion of the InR fragment into a bond of the reagent (e.g., I-I or S-S), and in the case where R ) C5H5, a change from η5- to η1-coordination appears to accompany oxidation. Other molecular indium(I) compounds follow suit. For example, the tris(pyrazolyl)borate derivative HB(3,5-tBu2Pz)3In is oxidized by elemental selenium to HB(3,5-tBu2Pz)3IndSe, a rare example of an InIII compound with a terminal IndSe bond, and by elemental sulfur to HB(3,5t Bu2Pz)3InS5 containing an InS5 cyclic unit.207 The In4 framework of [InR]4 [R ) C(SiMe3)3] is evidently able to dictate to some extent the course of oxidation. Thus, mild halogenation, for example, with C2Cl6 or Br2/AlBr3,198 gives rise to cage-like indium(II) products of the type R4In4X4 (X ) Cl or Br) with distinctly shorter In-In bonds than those in the parent compound. Oxidation can also be made to proceed to cluster compounds containing inequivalent In atoms giving a nonintegral overall oxidation state, for example, R4In4Br2,198 R3In3I2,199 or R4In4S.202 (iii) Reduction. Cyclic voltammetric studies208 on solutions of [InC(SiMe3)3]4 show behavior common to that of the gallium analogue, with reversible reduction at about -2 V vs Fe+/0. While the reduced gallium product displays an EPR spectrum suggesting the formulation [GaR]4•- [R ) C(SiMe3)3], with the unpaired electron delocalized over all four metal atoms, no EPR signal has been detected for the corresponding indium product, the true nature of which has yet to be discovered. The reduced tetrasupersilyltetragallane derivative Na2Ga4R4‚2THF (R ) SitBu3) has also been prepared by sodium reduction of either RGaCl2 or [GaR]4; X-ray analysis of single crystals discloses Na2Ga4 clusters in which the Na atoms play a far from innocent role.209 Whether indium follows gallium in this respect has not been established. (iv) Coordination chemistry. It is through their basic properties and isovalence with CO that InR monomers [R ) C5H5, C5Me5, or C(SiMe3)3] have perhaps excited most interest. Contreras and Tuck210 were first to note the formation of 1:1 complexes between cyclopentadienylindium(I) and BX3 (X ) F, Cl, Br, or Me), although no definitive evidence of structure exists. On the other hand, the crystal structure does establish that the 1:1 adduct formed with InI3 by the tris(pyrazolyl)borate indium(I) molecule In{η3-HB(3-tBuPz)2(5-tBuPz)} is indeed a mixed-valence compound with a direct InI-InIII bond (as well as involving η1-coordination of a 5-tBuPzH molecule at the InIII site).211 What is now certainly clear, however, is that InR monomers [R ) C(SiMe3)3 or Cp* ) C5Me5] are capable of replacing CO or other π-acid ligands to produce a variety of organotransition metal compounds in which InR may function as either a terminal or bridging unit.74,75 Examples include not only the homoleptic species M(InR)4 [M ) Ni or Pt; R ) C(SiMe3)3]72,212 but also compounds containing both CO or Cp* and InR ligands [R ) C(SiMe3)3 or Cp*]: (OC)4Mn(µ2-InR)2Mn(CO)4,213 (OC)3Co(µ2-InR)(µ2-CO)Co(CO)3 and (OC)3Co(µ2-InR)2Co(CO)3,214 (OC)3Fe(µ2-InR)n(µ2-CO)3-nFe(CO)3 (n ) 1-3),215,216 Fe3(CO)10(µ2-InR)2,216 [Cp(OC)Ni]2InR,217 Cp*Rh(InCp*)2(InCp*Cl2),218 Cp*Ru(InCp*){η2-In2Cp*2(µ2-Cl)},219 and (Cp*In)2Pd(µ2-InCp*)2Pd(µ2-InCp*)2Pd(InCp*)2.73 Evidence of π-type backdonation from the transition metal atom, M, to the InI moiety is to be found in (a) the comparative shortness of the M-InI bonds, (b) the ν(CO) wavenumbers of mixed carbonyl derivatives, and (c) the results of quantum chemical calculations.74,75,78

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Organic groups are by no means essential for InI molecules to function in this way, as revealed, for example, by the characterization of the complexes [HB(3,5-Me2Pz)3]InFe(CO)4 and [HB(3,5-Me2Pz)3]InW(CO)5; prepared from the InIII derivative [HB(3,5-Me2Pz)3]InCl2‚THF and either Na2Fe(CO)4 or Na2W(CO)5, these also feature short metal-metal bonds.220 Carbene-like InI heterocycles such as :In[η2{DippNC(CF3)}2CH]177a and :In[η2-(DippN)2CNCy2]178b may also be expected to bind to transition metal centers, if the behavior of analogous GaI compounds is any guide.177b (v) Decomposition. Organoindium(I) compounds decompose on heating. Although disproportionation may well occur initially, all the indium emerges ultimately as the free metal. Thus, cyclopentadienylindium(I) starts to decompose at ca. 100 °C, and by spontaneous decomposition in dry anisole containing polyvinylpyrolidone as a stabilizer, it can be made to act as a source of monodisperse metal nanoparticles.221 Decomposition of the same compound at elevated temperatures in the gas phase in the presence of PH3 or other reagents has also been turned to advantage for the metalloorganic vapor-phase epitaxy (MOVPE) growth of InP and related compounds.164 The basicity of (η5-C5H5)In has the merit of inhibiting polymerization of PH3 under these conditions, and at temperatures of ca. 600 °C and atmospheric pressure, the reaction system (η5-C5H5)In/PH3/H2 can be made successfully to deliver InP films, for example, in lattice-matched Ga0.47In0.53As/InP heterostructures.164,222,223

3.3. Indium(I) Compounds with Appreciable Ionic Character 3.3.1. Preamble The In+ cation has much in common with Cu+ in its susceptibility to oxidation and disproportionation. Like Cu+, it is appreciably more polarizable than its main oxidation product (i.e., Cu2+ or In3+), so soft or polarizable ligands, such as carbanions, better able to exploit its properties relative to those of the more highly charged cation, act to stabilize it against oxidation. By contrast, partnership with harder anions, such as Cl- or CF3SO3-, that draw more on Coulombic interactions, tilts the balance in favor of oxidation and disproportionation. Simple indium(I) compounds of this second class are characterized in the solid state by extended 3D or 2D networks bound by relatively strong cohesive forces. Even if the degree of ionic character falls well short of 100%, the compounds can justifiably be described in terms of cationic (In+) and anionic components. Several such compounds have been characterized well in the solid state. Best known are the indium(I) halides, InCl, InBr, and InI, which, despite their sparing solubility in solvents that do not promote rapid disproportionation, afford the principal entry point to indium(I) chemistry. Structural authentication of simple (as opposed to mixed valence) In+ derivatives has otherwise extended to the trifluoromethanesulfonate,203 tetrafluoroborate,224 and various ternary halogenometallate compounds,225-238 as listed in Table 5. In+ cations are also found in some ternary or other multicomponent solids, including InMo4O6,239 In6La10O6S17,240 InGaE2 (E ) Se or Te),241 and Pt2In14Ga3O8F15.242 Thus, InMo4O6, which is isomorphous with NaMo4O6, consists of infinite anion chains [Mo4O6-]∞ cross-linked to form channels where In+ cations reside in sites of square pyramidal coordination to oxygen. Likewise InGaE2 crystals are made up of onedimensional linear chains of edge-sharing GaE4 tetrahedra



Table 5. Preparation and Characterization of Compounds Containing Indium Exclusively as More or Less Well-Defined In+ Cationsa compound

how made

InCl

see text

InBr InI InBF4 InEF6 (E ) P, As, or Sb)

see text see text In metal + BF3 in anhydrous HF In metal + PF5 or InBF4 + EF5 (E ) As or Sb) in anhydrous HF Mg metal + molten InBr3 at 450 °C Cd metal + molten InBr3 at 450 °C InI + CdI2 heated together in appropriate proportions InX + SnX2 heated together in appropriate proportions

InMgBr3 InCdBr3 In4CdI6 InSnX3 (X ) Cl or Br)

InSn2X5 (X ) Cl, Br, or I)

InX + SnX2 heated together in appropriate proportions

In2-2xSn5+xCl12 (0 e x e 0.15)

InCl + SnCl2 heated together in appropriate proportions

In3SnX5 (X ) Br or I)

InX + SnX2 heated together in appropriate proportions

InCrBr3

Cr metal + molten InBr3 at 450 °C

InMnBr3

Mn metal + molten InBr3 at 450 °C

InFeBr3

Fe metal + molten InBr3 at 450 °C

In2ZrBr6

Zr metal + molten InBr3 at 450 °C

In3Ti2Br9

Ti metal + molten InBr3 at 450 °C

In2ThBr6

In metal + ThBr4 + molten InBr3 at 450 °C InCp* (Cp* ) C5Me5) or InCl + CF3SO3H in toluene InSO3CF3 + L in toluene

InSO3CF3 InSO3CF3‚L (L ) dibenzo[18]crown-6) InMo4O6

In metal + MoO3 + Mo pressed pellet heated to 895 °C

InGaE2 (E ) Se or Te), InAlTe2

In metal + Al or Ga + E heated to 550-650 °C

a

characterization

ref

X-ray, polymorphic, numerous other properties X-ray, numerous other properties X-ray, numerous other properties X-ray, barytes structure type X-ray powder, Raman

25, 248, 270 25, 265 224 260

X-ray, NH4CdCl3 structure type X-ray, NH4CdCl3 structure type X-ray, Tl4HgBr6 structure type

225 226 227

X-ray, DTA, phase studies; new ABX3 structure type with InI and SnII in 8-coordinate 1:5:2 environments X-ray, DTA, phase studies; NH4Pb2Cl5 (X ) Cl) or NH4Pb2Br5 (X ) Br or I) structure type X-ray, DTA, phase studies; disordered Th7S12 structure type accommodating a wide range of homogeneity X-ray, DTA, phase studies; low-temperature Tl3PbBr5 structure type attributed X-ray and neutron, NH4CdCl3 structure type, magnetic properties, band structure calculations X-ray, NH4CdCl3 structure type, magnetic properties, band structure calculations X-ray and neutron, NH4CdCl3 structure type, magnetic properties, band structure calculations X-ray, K2PtCl6 structure type, magnetic properties, band structure calculations X-ray, Cs3Cr2Cl9 structure type, magnetic properties, band structure calculations X-ray, magnetic properties, band structure calculations X-ray

228-230

X-ray, structure essentially that of an isolated [LIn]+[CF3SO3]ion pair X-ray, isomorphous with NaMo4O6, electrical conductivity measurements X-ray, TlSe structure type

25, 250, 272

229-232 229, 233

230, 231 234

235 234, 235

236 237 238 203 203 239 241

Compilation restricted to compounds whose structures have been wholly or partially established by diffraction measurements.

interspersed with chains of In+ ions. The novel oxyfluoride Pt2In14Ga3O8F15 is remarkable for the presence of not only highly positive [PtIn6]10+ octahedra, but also In+ ions. Rather better known for featuring In+ ions are mixed valence compounds such as InI[InIIIX4] (X ) Br or I), InI3[InIIICl6], InI2[InII2Br6], and InI3[InII2Br6]Br, but these will be reviewed separately in section 5. In other respects, In+ behaves like a heavy alkali-metal cation or, more obviously still, like its heavier group 13 homologue Tl+.25 In token of this, indium can take the place, partially or wholly, of alkali-metal or Tl+ cations in appropriate zeolites. By a solvent-free redox ion-exchange reaction with In metal at 623 K, for example, all of the Tl+ ions in fully dehydrated, fully Tl+-exchanged zeolite A can

be replaced by indium. X-ray studies of single crystals of the product imply, however, that the indium is present not just as In+ but also as In0, In2+, In3+, and the clusters [In3]2+ and [In5]8+.160,243 Incorporation of In+ ions into H-ZSM-5 has been achieved by thermal autoreductive ion exchange in mechanical mixtures of the solid zeolite and In2O3 under high vacuum at 840 K.244 Such zeolites are of interest not only for the nanosized clusters that they may support, but also as potential catalysts, for example, for the reduction of NO with hydrocarbons in the presence of oxygen.245 In the field of high-temperature superconductors, indium as In+ offers a less toxic alternative to thallium as Tl+ in thalliumbased cuprates, and indeed the materials (In0.3Pb0.7)(Ca0.8Y0.2)Sr2Cu2Oy and (Bi,In)Sr2(Gd0.6Ca0.4)Cu2Oy both


Figure 10. Representative coordination geometries of the InI centers in more or less ionic indium(I) compounds: (a) coordination of In+ by Br- (up to 4.50 Å) within InBr; (b) coordination of one of the In+ ions by Br- (up to 4.50 Å) within In[In2Br6]; (c) InF15 polyhedron within Pt2In14Ga3O8F15. Panels a and b reproduced with permission from ref 248. Copyright 1994 American Chemical Society. Panel c reproduced with permission from ref 242. Copyright 2005 American Chemical Society.

display superconductivity at temperatures near 60 and 30 K, respectively,246 although (In,Cu)Sr2YCu2O6+δ and (In,Cu)(Sr,Ho)2(Ho,Ce)2Cu2O8+δ, also with perovskite structures, do not.247 If In+ resembles Cu+ in its stabilization by polarizable ligands, the resemblance ends in its choice of coordination geometry. In its preference for environments of high coordination number, it comes closer to a heavy alkali-metal cation such as Cs+. However, the difficulty of differentiating clearly between primary and secondary metal-ligand contacts means that it is not always easy to specify unequivocally the coordination geometry of the InI center.25 How diffuse the environment can be is well illustrated in Figure 10 for InX7 (X ) Cl, Br, or I), InBr9, and InF15 polyhedra met in InX with the TlI structure type,248 orthorhombic In2Br3,248 and Pt2In14Ga3O8F15,242 respectively. A similarly complex coordination environment [made up of four close contacts ( InII > InIII; as may be expected, experimentally determined stretching force constants follow the reverse order.334 The molecules are photolabile, and UV irradiation results in decomposition yielding the corresponding InI compound InX and H atoms (eq 3). It is doubtful whether photodissociation is specific to the In-H bond; the observed behavior is more likely to reflect the relative mobilities of X and H fragments in a solid argon matrix,86 providing another example of how chemistry is controlled by the environment. It is interesting to note that photolysis of HInNH2 under these conditions gives rise to a significant secondary reaction affording H2InNH2,116 while HInPH2 suffers a tautomeric change, also to give an InIII product, namely, H2InPH, with the unpaired electron now localized mainly on the phosphorus atom.117 There is ample experimental evidence identifying mononuclear indium(II) species as intermediates in the oxidation or disproportionation of indium(I) compounds,40,41,135-137 and estimates of upper limits of -0.23 and -0.65 V have been made for the standard reduction potentials of the aqueous couples In(II)/In(I) and In(III)/In(II), respectively.137d Polarographic studies of complexes in which InIII is coordinated by a bidentate anionic ligand such as a dithiolate335 or tropolonate336 indicate that reduction proceeds in successive one-electron steps InIII f InII f InI f In0. However, the exact natures of these InII species remains mostly a matter of conjecture. In this context, the oxidative addition of an o-quinone to InX (X ) Cl, Br, or I) proves quite revealing.39-41,136,186 Thus, EPR measurements imply that both indium(I) and indium(III) semiquinone species are present in the reaction mixture formed by 3,5-di-t-butyl-obenzoquinone and InX, in keeping with the reaction scheme 11. A key step, it is inferred, is the dimerization of the

4.2. Mononuclear InII-Centered Molecules Sighting and identification of a genuine divalent indium compound have been realized so far only in a solid argon matrix environment at low temperatures. Here it is found that photoexcitation of In atoms from the 2P ground electronic state to the 2S or 2D excited state in the presence of HX molecules results in oxidative insertion into the H-X bond and the formation of a bent H-In-X molecule, where X ) H,114 Me,121 NH2,116 PH2,117 or OH.119 The product is formed initially in an excited electronic state and would almost certainly disintegrate in the gas phase to give an indium(I) compound and H atoms or X radicals. The matrix performs the vital functions of holding the indium(II) molecule in place, inhibiting the escape of would-be dissociation products, and facilitating relaxation to the ground electronic state

mononuclear InII semiquinone intermediate 25 to give an InIn-bonded molecule, which then disproportionates with halide


transfer to give the InI and InIII semiquinone products. Competing with dimerization is electron transfer in the InII intermediate, a change promoted by the presence of a strong donor such as phenanthroline. With a more strongly oxidizing quinone, for example, o-O2C6Y4 (Y ) Cl or Br), there is no sign of the semiquinone species, and electron transfer yielding InIII catecholate derivatives becomes the dominant reaction channel.307

4.3. Compounds Featuring an InII−InII Bond 4.3.1. Formation of the In−In Bond Associated with a mononuclear InII species •InXY (where X and Y are the same or different univalent substituents) is the characteristic reaction of dimerization to form an In-In bond in the diamagnetic molecule XYInInXY (eq 12). Since

the unpaired electron in •InXY is more or less localized on the metal, In-In bond formation is achieved at little expense to the In-X/In-Y bonds, and with little expectation of an appreciable activation barrier, the change is highly favorable in kinetic, no less than thermodynamic, terms. Quantum chemical calculations focusing mostly on the hydrides M2H4 of the group 13 elements337,338 identify potential energy minima corresponding to the classical D2h and D2d structures 26 and 27. One such calculation drawing

on nonlocal density functional theory (DFT)338 finds the D2d form of In2H4 with re(In-In) ) 2.78 Å to be 11 kJ mol-1 more stable than the D2h form with re(In-In) ) 2.83 Å and to be formed from 2 mol of InH2 (eq 12) with an energy change of -285 kJ mol-1 (cf. -530, -283, and -310 kJ mol-1 for M ) B, Al, and Ga, respectively). Wider exploration of the M2H4 hypersurface shows 27 to be the global minimum for M ) B, in keeping with the stable existence of B2X4 molecules (X ) F, Cl, Br, or I),23 but for the group 13 metals M ) Al or Ga337 (and presumably also M ) In), the global minimum adopts not this form but the tri-hydrogen-bridged structure 28. The latter can be described in terms of more or less Coulombic interaction between a cationic MI center and a tetrahedral, anionic MIIIH4 one, thereby implying disproportionation to a mixed valence form. The hydrides are merely paradigms for all M2X4 systems, and as noted elsewhere,25,40,41 a fine energy balance exists between a M-M-bonded structure akin to 26 or 27 and a charge-separated, mixed valence one akin to 28. Which form is favored depends not only on the substituent X but also on the phase and choice of solvent. The M-M bond is weakened by Coulombic repulsion between the partial positive charges residing on each of the metal atoms, a factor that is accentuated as the electronegativity of X increases. Conversely, any form of ligation that increases the electron density at the metal serves to strengthen the M-M bond. On the other hand, the charge separation implicit in the mixed

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valence structure is favored by the increased permittivity of the solid state or of a polar solvent. There is, in addition, a significant activation barrier to the interconversion of the M-M-bonded to the mixed valence structure, reflecting the ability of X to act as a bridging ligand between the two metal centers, which decreases in the order X ) halogen > hydrogen > organic group. In keeping with these general considerations, we find that InBr2 and InI2, the only known binary dihalides of indium,25 adopt mixed valence structures InI[InIIIX4] in the solid state.270,332 By contrast, In2X4 compounds with an In-In-bonded structure and tricoordinated metal atoms are confined to systems incorporating bulky organic or pseudo-organic substituents, for example, X ) CH(SiMe3)2,339 Trip,340 C6H2-2,4,6-(CF3)3,324 SitBu3,52,287,341 SitBu2Ph,52 or Si(SiMe3)3.342 Preservation of the InII-InII bond can otherwise be achieved only by expanding the coordination shell about, and increasing the electron density at, the metal atoms through the inclusion of neutral or anionic donor species, as in [C6H3-2,6-(CH2NMe2)2(Cl)In]2,343 [In2Br6]2-,270,344,345 and In2I4(PnPr3)2.346

4.3.2. Preparation and Physical Properties of InII−InII-Bonded Compounds Table 6 lists methods of preparation and characterization for the InII-InII-bonded compounds that have been reported to date with varying degrees of certainty; selected molecular structures are shown as formulas 29-42 (Chart 1). The earliest reports relied on circumstantial or limited spectroscopic evidence to deduce the presence of the In-In bond. In one of the first in 1962, Kochetkova et al.347 described the solid ammines In2X4‚nNH3 (X ) Br or I; n ) 6 or 8) formed by InX2 and gaseous ammonia and suggested that they should be formulated not as mixed valence compounds analogous to the parent halides, but as In-In-bonded species. Similar conclusions were reached and justified mainly on the grounds of Raman studies for a wide range of neutral complexes having the formulas In2X4‚2L (X ) Cl, Br, or I; L ) oxygen or sulfur base) and In2X4‚4L (X ) Cl, Br, or I; L ) nitrogen base or Me2SO).348 Definitive structural characterization of such a complex was not achieved, however, until 1984 when Tuck et al. showed the mixed halide adduct In2Br3I‚2TMEDA to have the structure 29 with an In-In bond measuring 2.775(2) Å.349 The year 1987 saw the characterization of the ethane-like [In2Br6]2- anion in the mixed valence compound having the empirical formula In2Br3.270 Two years later, Uhl et al. were the first to report the crystal structure of a tetraorganodiindane, namely, [(Me3Si)2HC]2In-In[CH(SiMe3)2]2, with a planar C2InInC2 skeleton, an In-In bond measuring 2.828(1) Å, and tricoordinated metal atoms.339 Three main methods have been used to form adducts of the diindium tetrahalides. The first involves direct action between the parent subhalide and a halide source or an organic donor, such as an ether, amine, sulfide, or phosphine (eq 13).259,277,297,347,348,350 The second depends on synpropor-

tionation between InX and InY3 (where X and Y may be



Table 6. Preparation and Characterization of In-In-Bonded Indium(II) Compounds compounda [(Me3Si)2HC]2In-In[CH(SiMe3)2]2 (Trip)2In-In(Trip)2 [C6H2-2,4,6-(CF3)3]2In-In[C6H2-2,4,6-(CF3)3]2 (tBu3Si)2In-In(SitBu3)2 (PhtBu2Si)2In-In(SitBu2Ph)2 [(Me3Si)3Si]2In-In[Si(SiMe3)3]2 [(Me3Si)2HC]2InIn[CH(SiMe3)2]2‚2RNC (R ) tBu or Ph) [Li(TMEDA)2] [{(Me3Si)2HC}2InIn{CH(SiMe3)2}2(C2Ph)] [(MeSi)2(NtBu)4]In-In[(NtBu)4(SiMe)2], 30 [(tBuNP)2(tBuN)2]In-In[(NtBu)2(PNtBu)2], 31 syn-[P2N2]In-In[P2N2] [P2N2 ) PhP(CH2SiMe2NSiMe2CH2)2PPh], 32 RIn-InR {R ) η3-[Me3SiNCH2]3(µ-In)CMe}, 33 (RS)2In-In(SR)2 (R ) CMe2Et or 2-naphthyl) R(Br)In-In(Br)R [R ) (Me3Si)2C(Ph)C(Me3Si)N], 34 [(Me3Si)3C](acac)In-In(acac)[C(SiMe3)3], 35 [{(Me3Si)3C}InX]4 (X ) Cl or Br), 36 [(Me3Si)3C]In(µ-O2CPh)2In[C(SiMe3)3], 37 (DAB)ClIn-InCl(DAB) (DAB ) ArNCHdCHNAr, Ar ) mesityl), 38 R(Cl)In-In(Cl)R [R ) HC(CMeNDipp)2], 39 R(Cl)In-In(Cl)R [R ) C6H3-2,6-(CH2NMe2)2], 40 (THF)RIn-InR(THF) [R ) 1,8-bis(trimethylsilylamido)naphthalene], 41 (tBu4pc)In-In(tBu4pc)‚2TMEDA (tBu4pc ) tetra-t-butylphthalocyanine) (TBSQ)XIn-InX(TBSQ) (X ) Cl or Br; TBSQ ) 3,5-di-t-butyl-o-benzosemiquinone) [Bu4N]2[In2X6] (X ) Cl, Br or I) K2[In2Br6] InI3[InII2Br6]Br InI2[InII2Br6] [Ph4P]2[In2Cl6] [CH(NMe2)2]2[In2Cl6] In2X4‚nNH3 (X ) Br or I; n ) 6 or 8) In2X4‚2L (X ) Cl, Br, or I; L ) 1,4-dioxane, tetrahydropyran, THF, or tetrahydrothiophene) In2X4‚2L (X ) Br or I; L ) Me2S) In2X4‚4L (X ) Cl, Br or I; L ) piperidine, piperazine or morpholine) In2X4‚4L (X ) Br or I; L ) py or Me2SO) In2X4‚2PEt3, In2X4‚2TMEDA, In2X4‚2TMEDA‚ArH (X ) Br or I; ArH ) C6H6 or toluene) InGaX4‚2L, [Bu4N]2[InGaX6] (X ) Cl or Br; L ) py, piperidine, piperazine, 1,4-dioxane, THF, or tetrahydropyran) In2I4‚6EtNH2, In2I4‚2bipy‚4EtNH2 In2X4‚4aniline (X ) Cl, Br, or I)

how madea

characterizationb

In2Br4‚2TMEDA + LiCH(SiMe3)2 in pentane/ether In2Br4‚2TMEDA + TripMgBr in Et2O InCl + LiC6H2-2,4,6-(CF3)3 in Et2O InBr + NaSitBu3 in THF; InN(SiMe3)2, InCp, InCp*, or (tBu3Si)2InX (X ) Cl or Br) + NaSitBu3 in alkane solution InN(SiMe3)2, InCp, or InCp* + NaSitBu2Ph in alkane solution InCl3 + LiSi(SiMe3)3 in pentane [(Me3Si)2HC]2InIn[CH(SiMe3)2]2 + RNC neat (R ) tBu) or in n-pentane (R ) Ph) [(Me3Si)2HC]2InIn[CH(SiMe3)2]2 + LiC2Ph in the presence of TMEDA in n-pentane/n-hexane [(MeSi)2(NtBu)4]InCl + Na naphthalide in THF InCl + [(tBuNP)2(tBuNLi‚THF)2] in toluene/THF syn-[P2N2]InCl + KC8 in Et2O

X-ray, vib, UV/vis, MS X-ray, NMR X-ray, NMR, MS X-ray, UV/vis, NMR, MS

2.828(1) 2.775(2) 2.744(2) 2.9217(11)

339 340 324 52, 287, 341

X-ray, UV/vis, NMR, MS

2.938(1)

52

X-ray, NMR X-ray, IR

2.8680(6) 342 2.8469(4) (tBu) 375

X-ray, IR, UV/vis, NMR

2.8534(4)

374

X-ray, NMR, MS X-ray, NMR X-ray, NMR, MS

2.768(1) 2.7720(4) 2.7618(12)

356 293 357

2.807(1)

140

In7Te10 ≡ 1/2InII2InIII12[Te-II]20 MIn5S6 ≡ MI[InII2]2InIII[S-II]6 (M ) K or Tl) MIn5S7 ≡ MIInII2InIII3[S-II]7 (M ) Na, K, or Tl), TlIn5Se7 MIn7E9 ≡ MI[InII2]2InIII3[E-II]9 (M ) Rb or Cs; E ) S or Se)

ref

InCl + MeC[CH2N(Li)SiMe3]3(dioxane)3 X-ray, NMR in dioxane electrolytic oxidation of In in the presence of RSH in NMR, Raman, MS MeCN solution; [Et4N]ClO4 supporting electrolyte InBr + RLi‚THF in toluene X-ray, IR, NMR, MS

2.728(4)

289

[{(Me3Si)3C}InCl]2 + Li(acac) in n-hexane/toluene [{(Me3Si)3C}In]4 + C2Cl6 or Br2/AlBr3 in toluene or n-hexane [(Me3Si)3C]3In3I2 + AgCO2Ph in toluene InCl + DAB in toluene

X-ray, IR, UV/vis, NMR X-ray, IR, UV/vis, NMR

376 198

X-ray, IR, NMR X-ray, IR, MS

2.7804(7) 2.823 (Cl) 2.835 (Br) 2.654(1) 2.7280(9)

InCl + LiR in THF RInCl2 + Li2[C4H4BNiPr2] in THF

X-ray, NMR X-ray, MS

2.8343(7) 2.7162(8)

292 343

InCl + Li2R‚4THF in THF

X-ray, NMR

2.7237(6)

355

(tBu4pc)InCl + Mg, I2, and TMEDA in THF

EXAFS, Raman, NMR

3.24

361

InX + TBQ/pyridine in toluene

EPR

[Bu4N]X + InX2 in xylene heat KBr + InBr3 + In to 450 °C heat In + InBr3, followed by sublimation heat In + InBr3, followed by sublimation InCl + [Ph4P]Cl in MeCN InCl + [CH(NMe2)2][GeCl3] in THF InX2 + overpressure of NH3 InX2 + donor

vib X-ray powder X-ray, Raman X-ray X-ray X-ray

187

360 309

136

Raman

350 344 345 270 353 354 347 348a

InX2 + donor in C6H6 or toluene; InX + TMEDA or PEt3 in toluene

vib, NMR

348b

InX + GaX3 + L or [Bu4N]X

Raman

352

InX2 + L (L ) bipy and/or EtNH2) InX2 + aniline; precipitated by Et2O

thermal analysis conductivity measurements on solutions IR, NMR X-ray, vib, NMR 2.775(2) X-ray, Raman, NMR 2.745(3) X-ray, IR, NMR, MS 2.7436(7)

297 277 351 349 346 309

X-ray

2.715(1)

281

X-ray powder X-ray X-ray, EXAFS X-ray, EXAFS

2.80 2.818(6) 2.741 2.760(5)

363 364 365, 366 366, 367

X-ray X-ray and thermal studies

2.763

368 369

In2XY3‚2TMEDA (X ) Cl, Br, or I; Y ) Br or I) InX + InY3 + TMEDA in toluene/CH2Cl2 I(Br)In-InBr2‚2TMEDA, 29 InI + InBr3 + TMEDA in toluene or CH2Cl2 In + nPr3PI2 in Et2O I2In-InI2‚2PnPr3 InBr + Imes in toluene Br2In-InBr2‚2Imes [Imes ) CN(Ar)C2H2N(Ar), Ar ) mesityl], 42 Cl2In-InCl2‚4THF InCl + Ph3PAuCl in THF InS ≡ InII2S2 InSe ≡ InII2Se2 In6S7 ≡ InIII2[InII2]2S7 In6Se7 ≡ InIII2[InII2]2Se7

r(In-In), Å

2.67(1) 2.707 2.702 2.727(1) 2.719(1)

Extended Solids

heat In + In2S3 heat In + Se at 800 °C heat In + In2S3 thermal decomposition of In2Se3 at 1000 °C under an Ar flow heat In + Te heat K + In + S or TlS + InS in stoichiometric proportions heat Na or K + In + S in stoichiometric proportions or Tl+/In+ exchange in In6S7 or In6Se7 heat MN3 + In2E3 in stoichiometric proportions at 599-764 °C

X-ray and thermal studies X-ray, HRTEM and thermal studies

369 2.7358(8) (Rb) 370 2.731(1) (Cs) (E ) S)


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Table 6 (Continued) compounda

how madea

In5E5X (E ) S or Se; X ) Cl or Br) ≡ heat In + elemental S or Se + InX3 in appropriate proportions at 500 °C InIInII2InIII2[E-II]5X-I

characterizationb X-ray, HRTEM

r(In-In) Å

ref

2.723(4) (S/Cl) 371 2.748(2) (Se/Cl) 2.7279(8) (S/Br) 2.739(2) (Se/Br) 2.630(1) 372, 373 2.862(2) 373

heat InPO4 + In2O3 after thermal reduction with H2 X-ray, Raman, ν(In-In) ) 195 cm-1 heat In2O3 + red P with I2 as a mineralizer at 800 °C; X-ray, IR, Raman, ν(In-In) ) 163 cm-1 crystals grown by chemical vapor transport in a temperature gradient a Trip ) C6H2-2,4,6-iPr3; TMEDA ) Me2NC2H4NMe2; Dipp ) C6H3-2,6-iPr2; TBQ ) 3,5-di-t-butyl-o-benzoquinone; py ) pyridine; bipy ) 2,2′-bipyridyl; Cp ) C5H5; Cp* ) C5Me5; acac ) MeCOCHCOMe. b X-ray ) crystal structure analysis (single crystal unless stated otherwise); vib ) vibrational spectroscopy; IR ) IR spectroscopy; Raman ) Raman spectroscopy; UV/vis ) UV/vis spectroscopy; NMR ) NMR spectroscopy; MS ) mass spectrometry; EXAFS ) EXAFS measurements; EPR ) EPR spectroscopy; HRTEM ) high-resolution transmission electron microscopy. In3(PO4)2 ≡ 1/2[InII2]3[PO4]4 In2OPO4 ≡ 1/2[InII2]InIII2O2[PO4]2

Chart 1

the same or different halogens, Cl, Br, or I) in the presence of the donor, for example, TMEDA (eq 14).39,349,351 In a

variation of this route, InX reacts with GaX3 in the presence of the donor L to give mixed metal derivatives of the type


InGaX4‚2L (X ) Cl or Br).352 The third method starts with an indium monohalide InX and proceeds by disproportionation on addition of the donor species, for example, Cl-, :CN(Ar)C2H2N(Ar) (Ar ) mesityl) or Ph3PAuCl in THF (eq 15).259,281,309,353,354 Indium metal is another possible source,

not only through its reaction on heating with InBr3 to produce the subhalides In4Br6270 and In5Br7345 (both containing the [In2Br6]2- anion) and, with the addition of KBr, K2[In2Br6],344 but also in its reaction with nPr3PI2 that affords the phosphine adduct In2I4‚2PnPr3.346 Diindium compounds carrying terminal bonds to C, Si, N, S, or halogen atoms, including homoleptic In2R4 compounds (where R is a bulky organic or silyl ligand), have been prepared in four ways: (i) metathesis between In2X4 adducts and the corresponding alkali metal derivative (e.g., eq 16);339,340 (ii) disproportionation and metathesis brought

about by the action of the corresponding alkali metal derivative on InX (e.g., eq 17);52,289,292,293,324,355 (iii) reduction

of an appropriate InIII precursor with or without metathesis (e.g., eqs 18 and 19);342,343,356,357 and (iv) electrolytic oxidation

of elemental In in the presence of a suitable ligand or ligand precursor (e.g., RSH) in acetonitrile solution.187 Diindium compounds normally form molecular solids at ambient temperatures. These have been characterized by their vibrational spectra, with the Raman spectra giving diagnostic evidence for the presence of the In-In bond through the appearance of strong scattering at 100-200 cm-1 identifiable, at least partially, with the ν(In-In) motion.25,53 Nuclear quadrupole resonance (NQR) measurements made on the dioxane complexes In2X4‚2C4H8O2 (X ) Cl or Br) are of


interest for what they imply about the extent of electron transfer from the ligands to the metal atoms.358 Most revealing, however, have been the results of X-ray analysis of single crystals that establish the existence of more or less discrete neutral or charged diindium species. The coordination number at each metal atom varies from 3 {as in In2[CH(SiMe3)2]4}, through 4 (as in the [In2Br6]2- anion), to 5 (as in 32). Although there are numerous similarities with analogous Al2 and Ga2 compounds, the tendency of the larger In atom to opt for higher coordination numbers is sometimes revealed. For example, the 3-fold coordination of the metal atoms in the digallium tetraamide 43359 gives way to 4-fold

coordination in the otherwise similar diindium compound 41 through the additional binding of a THF molecule to each metal atom.355 The conformation of the In2 assembly does not always comply with expectations of the lowest energy form of the free molecule. Thus, molecules of the type R2InInR2 with bulky η1-organo or silyl ligands R would be expected to adopt a staggered (D2d) E2In-InE2 skeleton (E ) C or Si) analogous to 27. In practice, the solid compounds reveal dihedral angles subtended by the two InE2 planes ranging from 94.1° [R ) C6H2-2,4,6-(CF3)3],324 through 48° (R ) Trip),340 to 6.7° [R ) CH(SiMe3)2].339 In the same vein, systems in which the metal atoms are each bound to not two but three terminal ligands are as likely as not to adopt an eclipsed conformation about the In-In bond and not the staggered conformation that the minimization of nonbonded repulsions and analogy with ethane might lead one to expect. Evidently the dictates of packing and secondary interactions in the crystal are quite capable of overriding the conformational preference of the free molecule. Compounds of this type are characteristically not only dinuclear in indium but also contain an In-In bond unsupported by any bridging ligand. Somewhat out of character therefore is the compound In2[C(SiMe3)3]2(O2CPh)2, 37, prepared by the reaction of [(Me3Si)3C]3In3I2 with AgCO2Ph in toluene.360 With terminal alkyl groups, this features an In2 unit bridged by two carboxylato groups with the chelate In2-η2-O2C fragments roughly orthogonal to each other. A second break with convention is afforded by compounds having the empirical formula (Me3Si)3CInX (X ) Cl or Br) prepared by mild halogenation of the indium(I) compound [(Me3Si)3CIn]4.198 These prove to be not dimers but tetramers, 36, with two InII2 units arranged orthogonal to each other to complete what is still recognizable as an In4 tetrahedron, four edges of which are spanned by halogen bridges. All the circumstantial evidence argues that the In-In bond is weak. The In-In distances measured for crystalline compounds span quite a wide range, from 2.702 Å for the [In2Br6]2- anion in InI2[InII2Br6]270 to 2.938 Å for In2(SitBu2Ph)4.52 Measuring 2.654 Å, the In-In bond in In2[C(SiMe3)3]2(O2CPh)2 with its two carboxylato bridges, 37, is even shorter.360 At the other extreme, an unusually long In-In bond (3.24 Å) exists, according to extended X-ray absorption


fine structure (EXAFS) measurements, in the complex In2(tBu4pc)2‚2TMEDA (tBu4pc ) tetra-t-butylphthalocyanine), where the metal atoms are each coordinated by the four nitrogen atoms of the phthalocyanine ligand and the two nitrogen atoms of a TMEDA molecule.361 These distances may be compared with estimates of re ) ca. 3.1 Å for the discrete In2 molecule in its 3Πu (0u-) ground electronic state99 with its dissociation energy of only ca. 74 kJ mol-1.132 It appears therefore that the length, and presumably the strength, of the metal-metal bond in diindium, as in digallium,362 compounds is controlled mainly by the nature of the substituents. This takes precedence, evidently, over both the coordination number at the metal centers and the net charge carried by the molecule {cf. [In2Br6]2- and In2(SitBu2Ph)4}. Steric effects undoubtedly play a part (most conspicuously for the phthalocyanine complex), but a number of other factors, such as charge separation, electrostatic repulsion, and rehybridization, need also to be taken into account.53 Density functional calculations carried out on digallanes of the type R2GaGaR2 indicate a Ga-Ga bond distance that follows the order R ) SiH3 > CH3 > H > NH2 > HNCHdCHNH, shortening overall by about 10%.362 According to population analysis, electronegative substituents such as amide exert a -I effect on the metal atoms, leading to a marked increase in the positive charge they carry. Far from leading to attenuation of the Ga-Ga bond, however, this has the effect of increasing the relative s-content of the Ga-Ga σ-bonding orbital, with a concomitant contraction of the bond. Many diindium compounds are strongly colored, the color itself depending on the substituents. Thus, tetraorganodiindium compounds are orange-red, whereas the tetrasilyl compounds In2(SitBu3)4 and In2[Si(SiMe3)3]4 are deep violet and ruby red, respectively. On the other hand, the carboxylate In2[C(SiMe3)3]2(O2CPh)2 is colorless, and whereas K2[In2Br6] is yellow, the adducts In2Cl4‚2THF and In2I4‚2PnPr3 are both colorless. Interestingly, too, the UV/vis absorptions of tetraorgano and tetrasilyl derivatives exhibit red shifts, which correlate with increasing dihedral angle for the E2InInE2 skeleton (E ) C or Si).53 As indicated in Table 6, In2 pairs are also to be found in the extended structures developed by the subvalent indium chalcogenides InE (E ) S or Se),363,364 In6E7 (E ) S or Se),365-367 and In7Te10.368 Thus, the monochalcogenides are isotypic with GaS, containing InII centers each linked to three chalcogen atoms and a second metal atom to complete a distorted tetrahedral environment.25 Similar ethane-like E3InInE3 units also feature in the other subvalent chalcogenides, which can be formulated as mixed valence compounds, namely, InIII2[InII2]2E7 and 1/2[InII2]InIII12Te20. The ternary chalcogenides MIn5S6 (M ) K or Tl),369 MIn5E7 (M ) Na, K, or Tl; E ) S or Se),369 MIn7E9 (M ) Rb or Cs; E ) S or Se),370 and In5E5X (E ) S or Se; X ) Cl or Br)371 also contain indium in more than one oxidation state, including InII2 units. While these units are often formulated as In24+, a more accurate description surely calls for a greater degree of delocalization, and the affinity of these systems to molecular diindium(II) compounds is suggested by In-In separations of 2.72-2.82 Å. A closer approximation to In24+ is probably to be found in the phosphate In3(PO4)2, the crystal structure of which consists of an anionic network of relatively well isolated PO43- tetrahedra balanced by unusually compressed In24+ cationic pairs with r(In-In) ) 2.630(1) Å.372 The phosphate In2OPO4 also features In24+ cations, but now

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with r(In-In) ) 2.862(2) Å and in company with InIII.373 Intense Raman scattering at 195 and 163 cm-1 for In3(PO4)2 and In2OPO4, respectively, has been identified with the ν(In-In) vibration.373

4.3.3. Reactions of InII−InII-Bonded Compounds Despite their potential, for example, as sources of other indium-indium-bonded derivatives, diindium compounds have not proved to be the equal of indium(I) compounds as synthetic precursors. They typically lack the robustness of their InI counterparts, and new, useful reaction pathways are always at risk from competition with decomposition, in which disproportionation is often the main principle. Their sphere of action is almost entirely limited to the solid state and nonaqueous media. Under aqueous conditions, disproportionation occurs, and the In-In bond in a species such as [In2Br6]2- (believed to exist in low concentrations in HBr solutions) is cleaved to give In(I) and In(III) products.253 Reactions into which diindium compounds enter have been shown to include (i) addition, (ii) metathesis, (iii) oxidation of InII to InIII, and (iv) disproportionation. (i) Addition. The tetraorgano or tetrasilyl diindium compounds with their tricoordinated metal centers, being coordinatively unsaturated, have the potential to act as weak, bifunctional Lewis acids. With an appropriate choice of base, In2[CH(SiMe3)2]4 has been shown to form either 1:1 or 1:2 adducts, which preserve intact the In-In bond. Thus, with the lithium alkynide LiCCPh in the presence of TMEDA, it forms the complex [Li(TMEDA)2][{(Me3Si)2HC}2InIn{CH(SiMe3)2}2(C2Ph)].374 Here the additional PhC2- ligand occupies a terminal site in the coordination sphere of one of the In atoms, so that one metal atom is tetracoordinated while the other remains tricoordinated. The In-In distance in the complex betrays an elongation of only 0.025 Å compared with the parent diindane. Attempts to prepare a similar product by the reaction of the same diindane with neopentyllithium gave only a mixture; the isolation and characterization of the InIII compound [(Me3Si)2HC]2InCH2tBu implied that addition is being frustrated by disproportionation.51 On the other hand, complexation does result when the diindane is treated with an isonitrile RNC (R ) tBu or Ph), this time to give the 1:2 adduct [(Me3Si)2HC]2(RNC)InIn(CNR)[CH(SiMe3)2]2.375 With 4-fold coordination of both metal atoms, this has either a staggered (R ) tBu) or an eclipsed (R ) Ph) ethane-like skeleton, the isonitrile ligands being, respectively, either trans or cis to each other. An In-In bond only about 0.02 Å longer than that in the parent diindane, combined with long In-C links (>2.40 Å) to the isonitrile molecules, testifies to the weakness of the acid-base interaction. Whereas the diindium compound gives no evidence of insertion into the In-In bond, the analogous dialane and digallane react, even at low temperatures, with insertion of one or two isonitrile carbon atoms into the metal-metal bonds.53 (ii) Metathesis. Hopes that the range of diindanes might be enlarged substantially through metathesis reactions have been largely dashed by their lability and the growing realization of how much their long-term existence at ambient temperatures depends on the nature of the ligands supporting the In-In bond. Metathesis has certainly played a part in the preparation of the tetraorganodiindanes In2R4 {eq 16, R ) CH(SiMe3)2339 or Trip340}, although the reactions are far from quantitative and InIII[CH(SiMe3)2]3 is a significant byproduct in the formation of In2[CH(SiMe3)2]4. Bulk is



obviously important to the organic group R in sustaining compounds of the type In2R4. That it is by no means the only factor is revealed, however, by the successful synthesis and isolation of In2[CH(SiMe3)2]4 but not of In2[C(SiMe3)3]4. Exchange also features in the reaction of [(Me3Si)3CInCl]4 with lithium acetylacetonate yielding [(Me3Si)3C](acac)InIn(acac)[C(SiMe3)3] (acac ) acetylacetonate).376 This retains an In-In bond with each metal atom terminally coordinated by the alkyl and a bidentate acac group, 35. By contrast, the reaction of [(Me3Si)3CInBr]4 with lithium diphenyltriazenide, LiN3Ph2, does not take this course but results in disproportionation with the formation of the indium(III) product (Me3Si)3CInBr(η2-N3Ph2).376 Likewise, simple metathesis is not favored in the reaction of [(Me3Si)3CInCl]4 with the diiron carbonylate Na2[Fe2(CO)8] in the presence of THF. Instead, the major product is [Na(THF)4][Fe2(CO)6(µ-CO)(µ-InR)2Cl] [R ) C(SiMe3)3], appearing to result from cleavage of the In-In bonds of the precursor and reduction (by the carbonylate anion) to form InIR molecules, which then exchange with two CO ligands of the diiron fragment.377 It can be viewed as a sodium chloride-THF adduct of the neutral Fe2(CO)6(µ-CO)(µ-InR)2 molecule; four THF molecules and two oxygen atoms of terminal CO ligands from different anions make up the coordination sphere of the Na+ cations to give a curious one-dimensional polymer in the solid state. (iii) Oxidation. Studies of the tetrasilyldiindane In2(SitBu3)4 (In2R4) indicate that oxidation is brought about by a dihalogen, dioxygen, AgF2, and elemental selenium.52 Products specifically identified are RInIIIX2 (X ) halogen, from the reactions with X2 and AgF2), ROH (from the reaction with O2), and the heterocubane-like tetramer [RInIIISe]4 (from the reaction with selenium). The tetraalkyldiindane In2[CH(SiMe3)2]4 suffers oxidation at the hands of propylene sulfide or Ph3PE (E ) Se or Te) with insertion of a chalcogen atom into the In-In bond to form InIII products of the type R2InEInR2 [R ) CH(SiMe3)2] where E ) S, Se, or Te.378 Competition between different reaction pathways often makes it difficult to determine exactly what those pathways are, and although indium(III) products are commonly observed, the simultaneous formation of elemental indium suggests that disproportionation, and not direct oxidation, is at work. (iv) Disproportionation. A diindane of the general type X2InInX2 is invariably prone to disproportionate in accordance with the equilibria formally represented in eq 20.

X2InInX2 h InX + InX3 h In+[InX4]-

(20)

In the case where X is a halogen, thermodynamic arguments suggest that the relatively weak In-In bond is much less significant than the stronger In-X bonds formed by InIII and the large lattice energy of solid In+[InX4]- in determining where the balance lies.25,40 It is these last two factors that are probably critical in causing the binary “dihalides” InBr2 and InI2 both to assume the mixed valence form In+[InX4]in the solid state.270,332 In the gas phase, too, the stronger binding of the InIII center and extra Coulombic energy is likely to favor the InI(µ-X)3InIIIX molecule (cf. 28). Addition of negative charge, in the form of one or more neutral or anionic donors, stabilizes the X2In-InX2 fragment by reducing the Coulombic repulsion between the metal centers, in keeping with the short In-In bonds displayed by species such as In2I4‚2PnPr3346 and [In2Br6]2-.270,344,345 On the other hand, what is formulated as “In2Cl4” disproportionates in

toluene in the presence of dibenzo[18]crown-6 with the formation of the complex Cl(dibenzo[18]crown-6)In‚InCl3.203 Mechanistically, the disproportionation can be viewed as involving X- transfer, whereas the reverse process involves oxidative insertion of InX into an InIII-X bond (eq 21).25,40

The presence of donor ligands, giving a closer approach to coordinative saturation of the InII centers of the diindium tetrahalides, tends then to raise the activation barrier to halide transfer. Such kinetic stabilization of the dimetallic form is likely to be less effective, however, for indium than for gallium with its less expansive coordination shell, and there is experimental evidence to support this. For example, the disproportionation represented by eq 22 appears to take place

[MII2X6]2- h [MIX2]- + [MIIIX4](X ) Cl, Br, or I)

(22)

in nonaqueous solution for M ) In (on the evidence of 115In NMR measurements), but not for M ) Ga.379 Tetraorgano and tetrasilyl diindanes, In2R4, are only weakly acidic and lend themselves less easily to stabilization through complexation. Instead they appear to owe whatever thermal stability they enjoy to the limited strength of the In-In bond and to the steric protection of the bulky substituents R. What may be additional, possibly decisive, factors are the deficiencies of R as both leaving and bridging groups. Compared with a halogen, say, R is capable of forming, at best, only weak bridges between two of the heavier group 13 metal atoms,25 and this may well introduce a substantially greater kinetic barrier to migration and disproportionation, as represented by eq 21. Disproportionation is apt to accompany other reactions of the diindanes. In an early report,347 it was noted that prolonged exposure of the ammine In2X4‚6NH3 (X ) Br or I) induces, first, disproportionation to InX‚2NH3 and InX3‚ 6NH3 and, subsequently, disproportionation of the In(I) species. Similarly, In2Br4‚2py is stable to disproportionation in pyridine (py) solution only at temperatures below -30 °C.348 Disproportionation is also the dominant pathway taken by the diindane In2[CH(SiMe3)2]4 in its response to the protic reagents PhCO2H380 and (PhCO)2CH2,381 elemental indium being formed together with RInIII(µ-CO2Ph)4InIIIR and R2InIII[η2-{OC(Ph)}2CH] [R ) CH(SiMe3)2], respectively. Thermolysis of a diindane leads ultimately to the formation of indium metal. In the case of In2(SitBu3)4, however, heating to ca. 100 °C in boiling heptane yields tBu3SiH and the dodecaindane In12(SitBu3)8,382 a rare example of an indium “metalloid” cluster.6-8 Further details of this product are deferred for discussion in the following section.

5. Mixed or Intermediate Valence Indium Compounds 5.1. Preamble In this last section, we are concerned with an increasingly populous set of subvalent indium compounds that are relatively heterogeneous in character but each sharing the common feature of incorporating indium atoms not only in two or more different environments but also in different


oxidation states. The compounds, as listed in Table 7, range from classical mixed valence species such as InI[InIIIX4] (X ) Br or I),270,332 through distinct units that approximate to cationic clusters, for example, In35+ and In57+ as found in the extended solids In4E3 (E ) Se or Te)383,384 and In5Mo18O28,385 respectively, to neutral clusters as represented by In8Ar4 (Ar ) C6H3-2,6-mesityl2),386 In8(SitBu3)6,341 and In12(SitBu3)8.382 At one extreme, we thus have compounds that can reasonably be identified as belonging to class I of the Robin-Day classification of mixed valence compounds,387 with metal ions coexisting in ligand fields of very different symmetry and strength, valences firmly trapped at the different metal centers, no evidence of either electronic conduction or distinct mixed valence electronic transitions in the visible region, and spectroscopic properties consistent with the presence of the constituent ions, for example, In+[InX4]-. At the other extreme, metal-metal bonding between the different centers is evident. In addition, the clusters may be either homonuclear, as with the examples cited above, or heteronuclear, for example, [PtIn6]10+,242,388-390 [GeIn4]8+,391 In4S in [(Me3Si)3C]4In4S,202 In3I2 in [(Me3Si)3C]3In3I2,199 Au3In3 in (dppe)2Au3In3Cl6(THF)3 (dppe ) Ph2PC2H4PPh2),281 and In9P10 in In9(PPh)4(P2Ph2)3Cl7(PEt3)3.392 Table 7 concentrates on compounds that have been isolated and characterized more or less reliably at ambient temperatures and so does not include naked Inn clusters (2 < n e 200)134 or species such as InE3 and InE5 (E ) P393 or As394) belonging to the high-energy regime and as yet but thinly detailed by experiment. As indicated at the outset of this review, the table does not list the many examples of binary, ternary, or higher order intermetallic phases in which indium is alloyed with alkali, alkaline earth, or rare earth elements.3,4,25-32 Here the indium, which must assume a negative oxidation state, may be found in more or less isolated polyatomic units, for example, distorted tetrahedral In48- in Na2In,395 square-pyramidal In59in La3In5,396 and In117- (having the form of a compressed pentacapped trigonal prism) in K8In11.397 More often, however, it forms extended anionic networks, of which the twodimensional corrugated In9 layers in Li2Y5In9 are typical;398 these are themselves made up of two types of squarepyramidal In5 units linked by butterfly-like In4 units. Whereas CaIn contains isolated square In48-, SrIn and BaIn contain three bonded In2- ions connected to form, respectively, layers and ladders.399 The chemical bonding and electron count for most, but not all, of these intermetallic compounds can be explained by the Zintl-Klemm concept, as well as Wade’s rules. Many ternary phases including indium, a rare earth, and a transition metal have also been characterized,400 and with these comes the added variety of not just homonuclear anionic clusters and networks, as in LnIrIn2 (Ln ) lanthanide), where the indium forms a three-dimensional network of corner-sharing In4 tetrahedra,401 but also heteronuclear units, as in Yb2IrIn8 (composed of layers of YbIn12 cuboctahedra and “stuffed” InIn8 cubes)402 and Ln4Pt10In21 (with Ln ) lanthanide and having a three-dimensional Pt10In21 network).403 Nor is this the limit, for ternary and quaternary alkaline earth indium nitrides are also found to include polyanionic indium clusters, for example, tetrahedral In4 units in [M19N7][In4]2 (M ) Ca or Sr)404 but butterflylike ones in Ba14Cu2In4N7,405 and In5 and In8 clusters in Ba19In9N9,406 or extended networks, for example, chains of edgesharing In4 tetrahedra in Ba8Cu3In4N9,407 and zigzag chains of In in Ca2InN.408 Particularly striking is the stuffed In74

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building block (having the same conformation as the regular fullerene C74),409 which with the penultimate Na39 sphere is illustrated in Figure 12. Appearing in ternary phases of the type Na96In97M2 (M ) Ni, Pd, or Pt), this is the outer shell of a multiply endohedral unit constituted as Ni@In10@Na39@In74. Such an outline, illustrated by examples drawn from recent research, gives some indication of the scope of this particular topic and the dangers to both length and focus of trying to do justice to it within the framework of the present account. In fact, the preparation (mainly by combination of the relevant metals) and physical characterization of indides has proceeded almost independently of any other area of indium chemistry. It is tempting, in the circumstances, to speculate on the profitability of a greater degree of interaction, such as might lead, for example, to the isolation of discrete indium clusters in more tractable forms. For the present, then, we treat the subject of mixed or intermediate valence indium compounds, in which the oxidation state of the indium is judged to fall between 0 and +3, under the three subheadings employed in Table 7, namely, (i) compounds with discrete In or In2 centers, (ii) homonuclear and heteronuclear cationic In clusters, and (iii) homonuclear and heteronuclear neutral or anionic In clusters.

5.2. Compounds with Discrete In or In2 Centers Here we are concerned primarily with indium halides, chalcogenides, and oxysalts. Thermal synproportionation involving appropriate InIII and InI compounds or an InIII compound and In metal is the principal method of synthesis, supplemented by controlled direct reaction of the elements in some cases (e.g., InI2, InTe, and In7Te10) or reduction of an InIII compound {e.g., to produce [InI(C6D6)][(Me3Si)3CInIIII3]410 and MIn7E9 (M ) Rb or Cs; E ) S or Se)},370 or electrolytic oxidation of In metal {e.g., to produce “In(O2CMe)2”}331 in others. The phosphates In2P2O7 and In2OPO4 have been prepared by heating a mixture of InPO4 and InP in the first case411 and of In2O3 and red phosphorus in the second,373 I2 being used as a mineralizer. Some compounds that may themselves be either mixed valence InI/ InIII or In-In-bonded species in the solid state form adducts with donors, such as an arene or crown ether, able effectively to coordinate InI {as in [(mesitylene)2InI][InIIIBr4]412,413 and Cl3InIIIrIn(dibenzo[18]crown-6)Cl203}.

5.2.1. Halides Despite some earlier conflicting results, detailed studies of the systems In/InX3 (X ) Cl, Br, or I) appear now25,270,298,332,414-420 to have established the existence of stable solid phases other than InX with the following compositions: In2Cl3, In5Cl9, In7Cl9; InBr2, In2Br3, In4Br7, In5Br7, In7Br9; and InI2. There are thus significant variations from halogen to halogen. For example, InBr2 and InI2 are both well characterized as InI[InIIIX4]270,332 and therefore part of a classical tale in main group chemistry that began with the corresponding gallium dihalides, but InCl2, unlike GaCl2, appears not to exist. The pseudohalide In(CN)2 may well be formulated in a similar way, although it has been but thinly characterized,421 while a compound with the formula In2.24(NCN)3 unquestionably contains both InI and InIII.422 In2Cl3 and In2Br3 may have the same composition, but they differ in that the former is most aptly formulated as InI3[InIIICl6]414 and the latter as InI2[InII2Br6].270 By contrast, the In-InI3 system gives no hint of any intermediate phases other than



Table 7. Preparation and Characterization of Mixed Valence Indium Compounds compounda In2Cl3 ≡ InI3[InIIICl6] In5Cl9 ≡ InI3[InIII2Cl9] In7Cl9 ≡ InI6InIIICl9 InBr2 ≡ InI[InIIIBr4] In2Br3 ≡ InI2[InII2Br6] In4Br7 ≡ 1/2InIII3InI5Br14 In5Br7 ≡ InI3[InII2Br6]Br In7Br9 ≡ InI6InIIIBr9 InI2 ≡ InI[InIIII4] (mesitylene)2In2Br4 ≡ [(mesitylene)2InI][InIIIBr4] LIn2Br4 ≡ [LInI][InIIIBr4] (L ) [2,2]-paracyclophane) (Me3Si)3CIn2I3‚C6D6 ≡ [InI(C6D6)][(Me3Si)3CInIIII3] L′In2X4 ≡ [L′InI][InIIIX4] (L′ ) dibenzo[18]crown-6; X ) Cl, Br or I) L′′In2X4 ≡ [L′′InI][InIIIX4] (L′′ ) cyclam; X ) Br or I) I3InIII‚InI{η3-HB(3-tBuPz)2(5-tBuPz)}‚η1-5-tBuPzH Cl3InIII‚InI(dibenzo[18]crown-6)Cl In(CN)2 ≡ InI[InIII(CN)4] In2.24(NCN)3 ≡ InIII1.89InI0.34[NCN]3

how madea

419 25, 298, 332, 420 25, 164, 412, 413

InBr2 + L in mesitylene

X-ray, IR; structure as in 44

25, 164, 424

byproduct of the reaction of InI3 with (Me3Si)3CLi in toluene at ca. 0 °C InX2 + L′ in benzene

X-ray

410 NMR

425

InX2 + L′′ in benzene

IR, Raman, 1H NMR

425

X-ray

211

X-ray; structure as in 45 IR X-ray

203 421 422

X-ray, Raman, 31P-MAS NMR X-ray, IR, Raman

411

InI3 +

HB(3-tBuPz)3-

368 369

heating MN3 + In2E3 in stoichiometric proportions heating In + S or Se + InX3

X-ray, HRTEM, thermal studies X-ray, HRTEM

370

heating In + In2Te3 + InX3 heating La2O2S + In2S3

phase studies, X-ray X-ray

427 240

heating In + Te heating In + In2S3; thermal decomposition of In2Se3 heating In + Te heating the elements or TlS + InS in stoichiometric proportions or Tl+/In+ exchange in In6S7 or In6Se7

In5Mo18O28 ≡ [In5]7+[Mo18O28]7-

In11Mo40O62 ≡ [In5]7+[In6]8+[Mo40O62]15Na23In5O15 ≡ Na+23[In5]7+O15

In10Si12Al12O48‚In

373

X-ray X-ray, thermal studies

InTe ≡ InI[InIIITe2] In6E7 ≡ InIII2[InII2]2E7 (E ) S or Se)

In3Mo11O17 ≡ 1/2[In6]8+[Mo22O34]8-

25, 270, 298, 417 25, 270 418 25, 345

25, 426 365-367

heating In + In2(SO4)3 at T > 300 °C

GeIn4E4 ≡ [GeIn4]8+E4 (E ) S or Se) SnIn4E4 ≡ [SnIn4]8+E4 (E ) S or Se)

IR, Raman,

1H

25, 414 25, 414, 415 25, 416

IR, Raman, MS, thermogravimetric studies chemical and X-ray phase analysis, DTA, IR, Auger spectroscopy X-ray X-ray, EXAFS

InSO4 ≡ InI[InIII(SO4)2]

In4E3 ≡ InI[In3]5+E3 (E ) Se or Te)

ref

A. Compounds with Discrete Metal or Dimetal Centers phase studies, X-ray heating InCl + InCl3 phase studies, X-ray heating InCl + InCl3 heating InCl + InCl3, InCl + SnCl2, or InCl + phase studies, X-ray In2Cl3 phase studies, cryoscopy, Raman, X-ray heating In or InBr + InBr3, or In + Br2 phase studies, cryoscopy, X-ray heating In + InBr3 heating In + InBr3, followed by slow cooling X-ray, theoretical studies phase studies, X-ray heating In + InBr3 and sublimation in a temperature gradient heating InBr + InBr3 X-ray phase studies, Raman, X-ray heating In or InI + InI3 in xylene, or In + I2 InBr2 + mesitylene X-ray

“In2Cl4” + dibenzo[18]crown-6 in toluene heating In under a mixture of HCN + H2 heating InBr + NaCN at 400 °C followed by chemical transport In2P2O7 ≡ InIInIII[P2O7] isothermal heating of InPO4 + InP at 800 °C with I2 as a mineralizer In2OPO4 ≡ 1/2[InII2]InIII2O2[PO4]2 heating In2O3 + red P at 800 °C with I2 as a mineralizer; crystals grown by chemical vapor transport In(O2CMe)2 ≡ InI(O2CMe)‚InIII(O2CMe)3 electrolysis of In in acetic acid

In7Te10 ≡ 1/2InII2InIII12Te20 MIn5S6 ≡ MI[InII2]2InIIIS6; MIn5E7 ≡ MIInII2InIII3E7 (M ) Na, K, or Tl; E ) S or Se); Tl3In5S8 ≡ TlI3InIInIII4S8 MIn7E9 ≡ MI[InII2]2InIII3E9 (M ) Rb or Cs; E ) S or Se) In5E5X ≡ InIInII2InIII2E5X (E ) S or Se; X ) Cl or Br) In3Te3X ≡ InII2Te2‚InIIITeX (X ) Br or I) In6La10O6S17 ≡ InIInIII5LaIII10O6S17

characterizationb

B. Homonuclear and Heteronuclear Cationic Clusters heating elemental In + E at 600-700 °C, followed X-ray by directional freezing down to 400-500 °C heating elemental In and Ge with E X-ray; contains GeIn4 tetrahedra heating elemental In and Sn with E X-ray, contains SnIn4 tetrahedra; SnIn4S4 believed originally to be In5S4 heating Mo + MoO2 + In2O3 pellets at 1200 °C X-ray, electrical and magnetic properties; oligomeric molybdate clusters built from five trans-edge-sharing MoO6 octahedra and linked by almost linear In6 units contained in structural channels heating In + Mo + MoO2 at 1150 °C X-ray, electron microscopy; polymorphic; short chains of four trans-edge-sharing MoO6 octahedra cross-linked by nearly linear In5 units contained in structural channels heating In + Mo + MoO2 at 1050-1100 °C X-ray, electrical and magnetic properties; chains of edge-sharing MoO6 octahedra cross-linked by chain-like In5 and In6 units reaction of NaIn + Na2O2 + Na2O X-ray, electron microscopy, theoretical calculations; contains tetrahedral [InIn4]7+ units in all probability X-ray; zeolite structure includes In0, InI complete redox exchange of In for + Tl in fully dehydrated zeolite A (Tl12Si12Al12O48) and InII, with evidence of bent [In3]2+ and tetrahedral [In5]8+ polycations

331 428

371

383, 384 391 432, 433 435

385

434

429

430


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Table 7 (Continued) compounda

how madea

PtIn7F13 ≡ InIII[PtIn6]10+F13 Pt2In14Ga3O8F15 ≡ InI2[PtIn6]10+2GaIII3O8F15

heating Pt + In + InF3 at 750 °C heating Pt + In + InF3 + Ga2O3 at 600 °C

PtIn6(GaO4)2 ≡ [PtIn6]10+[GaO4]2 PtIn6(GaO4)2-x(InO4)x (0 e x e 1) PtIn6(GeO4)2O ≡ [PtIn6]10+[GeO4]2O PtIn6(GaO4)2-x(GeO4)xOx/2 (0 e x e 2) In4(Trip)6‚3OEt2 ≡ In0[InII(Trip)2]3‚3OEt2 In8(C6H3-2,6-mesityl2)4 ≡ In04InI4(C6H3-2,6-mesityl2)4 In8(SitBu3)6 ≡ In02InI6(SitBu3)6 In12(SitBu3)8 ≡ In04InI8(SitBu3)8 R3In3I2 ≡ InIInII2R3I2 [R ) C(SiMe3)3] R4In4Br2 ≡ InI2InII2R4Br2 [R ) C(SiMe3)3]

[Li(THF)3][R3In3Br3] ≡ [Li(THF)3]+[InIInII2R3Br3][R ) C(SiMe3)3] R4In4S ≡ InI[InIInII2]R4S [R ) C(SiMe3)3] [H(quin)2][In5Br8(quin)4] ≡ [H(quin)2]+[{In5}VIIBr8(quin)4](quin ) quinuclidine) [MeC{CH2NSiMe3}3In2]2 ≡ [MeC{CH2NSiMe3}3(µ-InI)InII]2 In9(PPh)4(P2Ph2)3Cl7(PEt3)3 ≡ InIII3[InII2]3(PPh)4(P2Ph2)3Cl7(PEt3)3 (dppe)2Au3In3Cl6(THF)3 ≡ (dppe)2Au02AuIInIInII2Cl6(THF)3 (dppe ) Ph2PC2H4PPh2) [L2Au][L2Au3In3Br7(THF)] ≡ [L2Au02AuIInIInII2Br7(THF)](L ) Ph2PC2H4PPh2)

characterizationb

X-ray, magnetic properties; contains PtIn6 octahedra X-ray, magnetic properties, electronic structure calculations; contains PtIn6 octahedra heating Pt + In + Ga2O3 + In2O3 at 947 °C X-ray, electrical and magnetic properties, electronic diffuse reflectance; contains PtIn6 octahedra heating Pt + In + InF3 + GeO2 + X-ray, magnetic properties, electronic structure calculations; Ga2O3 at 947 °C contains PtIn6 octahedra C. Homonuclear or Heteronuclear Neutral or Anionic Clustersc reduction of Trip2InInTrip2 with X-ray; molecule with trigonal planar InIn3 core Li powder suspended in Et2O at -78 °C InCl + LiC6H3-2,6-mesityl2 in X-ray, 1H and 13C NMR, UV/vis; molecule built on THF at ca. -78 °C distorted In8 cube, 46 InCp* + NaSitBu3 in pentane X-ray, 1H, 13C, and 29Si NMR; molecule built at -78 ˚C; In2(SitBu3)4 also formed on In8 cube stretched along one body diagonal, 47 decomposition of In2(SitBu3)4 in X-ray, 1H, 13C, and 29Si NMR; molecule heptane at ca. 100 °C; tBu3SiH built on In12 core composed of two distorted In6 also formed octahedra, 48 In4R4 + AlI3/I2 in n-hexane X-ray, 1H and 13C NMR, IR, UV/vis; central In3I2 cluster (see Figure 9) In4R4 + 1,2-C2H4Br2 in toluene at 60 °C X-ray, 1H and 13C NMR, IR, UV/vis; central In4Br2 cluster with one face and an adjoining edge of a roughly tetrahedral In4 unit spanned by µ3-Br and µ2-Br atoms, respectively (see Figure 9) byproduct of the reaction of InBr X-ray, 1H and 13C NMR, IR; central In3Br2 cluster with a bent In3 unit the terminal atoms of with LiR‚2THF in toluene which are bridged by 2 Br atoms, 49 X-ray, 1H and 13C NMR, cryoscopy, IR, UV/vis; In4R4 + propylene sulfide in hexane at 60 °C central In4S core with S atom capping one face of a distorted In4 tetrahedron (see Figure 9) X-ray, 1H NMR, IR, MS; anion has central controlled decomposition of quin‚InH3 in Et2O in the presence of tetrahedral InIn4 unit with 2 Br atoms LiBr at -30 °C and one quin attached to each terminal In atom [MeC{CH2N(Li)SiMe3}3(dioxane)3] X-ray, 1H and 13C NMR; molecule contains + InCl in dioxane InII-InII central bond and InI atoms bridging N atoms of the ligand, 33 InCl3 + PEt3 + PhP(SiMe3)2 in THF, X-ray, IR, UV/vis, theoretical calculations; with elimination of Me3SiCl compound made up of an In9P10 polyhedron, 50 X-ray, 31P NMR, 197Au Mo¨ssbauer, vis emission; Au3 Ph3P‚AuCl + InCl + dppe in THF triangle with one edge bridged by an In atom and capped above and below by In atoms, 51 X-ray, 31P and IH NMR, MS, theoretical calculations; Ph3P‚AuBr + InBr + L in THF structure analogous to that of 51

ref 388 242 389 390

340 386 341 382 199 198

290 202 196 140 392 281 282

a Trip ) C H -2,4,6-iPr ; Cp* ) C Me . b X-ray ) X-ray structure determination; neutron ) neutron diffraction; IR ) IR spectroscopy; Raman 6 2 3 5 5 ) Raman spectroscopy; UV/vis ) UV/vis spectroscopy; NMR ) NMR spectroscopy; MAS ) magic angle spinning; MS ) mass spectrometry; DTA ) differential thermal analysis; HRTEM ) high-resolution transmission electron microscopy. c Excluding Zintl and related phases.4,25-30

Figure 12. The fullerene-like In74 cluster (blue) and endohedral Na39 shell (red) connected to equivalent exohedral Na (green ellipsoids) of Na96In97Ni2; the In10Ni core is not shown. Reprinted with permission from ref 28a. Copyright 2000 Wiley-VCH.

InI and InI2. Differences between the chlorides and bromides seem to depend on the preference shown by chloroindium(III) species for 6-fold rather than 4-fold coordination, with the Coulombic advantages that flow from this.25 Only with the solids In7X9 (X ) Cl or Br) is there congruence between the two sets of compounds. These are best described as InI6InIIIX9416,419 and adopt a distorted NaCl-type structure similar to that of R-InCl, featuring InI3 triangles and a slightly distorted bisdisphenoid coordination shell about each InI.

Hexacoordinated InIII is also found, in association with InI, in In5Cl9, this time as the cofacial bisoctahedral anion [In2Cl9]3-.415 By contrast, tetracoordinated InIII and InII are the preferred forms accompanying InI in the bromides InBr2, In2Br3, and In5Br7 (InI3[InII2Br6]Br),270,345 although In4Br7 features two InIII sites, one with roughly tetrahedral and the other with roughly octahedral coordination.418 Tetraiodoindium(III) environments appear also to be preferred, at least on the evidence of the crystal structures adopted by both InI2332 and β-InI3.25 Intriguingly, although these intermediate indium halides are, to judge by their structures and spectroscopic properties, class I mixed valence compounds,387 a core photoelectron emission study of the indium chlorides InCl, In2Cl3, In5Cl9, InCl3, and what was purported to be InCl2 gave no hint of indium in more than one oxidation state, this despite an apparent 2-3 eV increase in the In 3d and 4d binding energy with the change from InI to InIII.423 There was no evidence either for In satellite peaks or skewing associated with the attainment of other electronic states (“shakeup” processes) or relaxation in the ionized product state. However, these features are judged to be normal for post-transition metal halides where the significantly greater decrease in metal radius caused by oxidation tends to counteract the expected increase in core binding energy.


Indium(I) shares with gallium(I) and thallium(I) a susceptibility to coordination by arene molecules such as benzene, mesitylene, and [2,2]-paracyclophane, the stability of the resulting complexes varying in the order GaI > InI > TlI.413 Hence InBr2 adds two molecules of mesitylene or one molecule of [2,2]-paracyclophane (L) to form [(mesitylene)2InI][InIIIBr4]412,413 or [LInI][InIIIBr4],424 respectively. The structure of the latter, as represented in 44, exemplifies the


[InII2]E7; M ) Na, K, or Tl; E ) S or Se),369 and MIn7E9 (MIInIII3[InII2]2E9; M ) Rb or Cs; E ) S or Se).370 The MIn5E7 phases adopt the In6S7 structure type, and in all these compounds, the In2 pairs occur as ethane-like In2E6 fragments. InI, InII2, and InIII all coexist in mixed chalcogenide halides having the composition In5E5X (E ) S or Se; X ) Cl or Br) and with two different structure types that comply most aptly with the formulation InIInIII2[InII2]E5X.371 Both InII2 and InIII occur in In3Te3X (X ) Br or I).427

5.2.3. Oxysalts

η6-mode of coordination of the C6 ring, also adopted in both the mesitylene complex and the benzene complex [InI(C6D6)][(Me3Si)3CInIIII3]411 and characteristic of this type of complex.413 The spectroscopic properties of crown ether and cyclam In2X4 complexes (X ) Cl, Br or I) have been read as indicating that these, too, are to be similarly formulated, namely, [LInI][InIIIX4] (L ) dibenzo[18]crown-6 or cyclam).425 Interestingly, though, the solid 1:1 In2Cl4 complex of dibenzo[18]crown-6 takes a rather different form, namely, Cl3InIIIrInI(dibenzo[18]crown-6)Cl, 45.203 This is only the second example of an authenticated compound featuring a coordinate InIIIrInI link that measures 2.7020(12) Å, the first being the tris(pyrazolyl)borate derivative I3InIIIrInI{η3HB(3-tBuPz)2(5-tBuPz)}‚η1-5-tBuPzH211 with an In-In distance of 2.748(4) Å. Such coordinate bonds thus lie at the short end of the range associated with normal InII-InIIbonded molecules (2.702-2.938 Å).

5.2.2. Chalcogenides Just how fine a balance exists between the options of InII-bonded and mixed valence InI/InIII structures is demonstrated by the finding that, while InS and InSe are InII compounds with metal-metal bonds, InTe resembles TlS and TlSe in being more appropriately assigned a mixed valence formulation InI[InIIITe2].25,426 InIIITe4 tetrahedra share edges to form chains with the InI lying between the chains and surrounded by a distorted cube of eight Te atoms. A phase richer in indium, such as In4E3 (E ) Se or Te), contains In3 clusters, but those in the composition range In/E ) 1:11:1.5 are made up of InIII and InII2 units. Such is the case with In6E7 (E ) S or Se) and In7Te10, which can be formulated as InIII2[InII2]2E7365-367 and 1/2InIII12[InII2]Te20,368 respectively. The same indium units also feature in the ternary phases KIn5S6 (KIInIII[InII]2S6),369 MIn5E7 (MIInIII3InII

Very few simple oxysalts that are mixed valence in indium have been structurally authenticated. Materials with the compositions In(O2CMe)2 and InSO4 have been prepared, the first by electrolysis of indium in acetic acid331 and the second by heating a mixture of elemental indium and In2(SO4)3.428 On the (admittedly limited) evidence of spectroscopic and thermal analysis, the acetate is thought to be a mixed salt InIO2CMe‚InIII(O2CMe)3, whereas the sulfate is credited with the formulation InI[InIII(SO4)2]. Only in the case of the phosphates In2P2O7411 and In2OPO4373 have spectroscopic measurements been supplemented by single-crystal X-ray analysis. Hence the former has been shown to consist of more or less well isolated In+, In3+, and [P2O7]4- ions; the In3+ ions are coordinated roughly octahedrally by O atoms at distances of 2.09-2.16 Å, while the In+ ions are surrounded unsymmmetrically by 10 O atoms at distances ranging from 2.82 to 3.41 Å. This contrasts with the oxide phosphate, which can be represented as 1/2InIII2[InII2]O2(PO4)2. Here the crystal is characterized by a three-dimensional network of PO4 tetrahedra and In2O10 groups formed by edgesharing InIIIO6 octahedra. Channels within the network accommodate relatively discrete In24+ cations.

5.3. Homonuclear or Heteronuclear Cationic In Clusters Extended anionic frameworks composed of chalcogenide, halide, oxide, halogenometal, or oxymetal units may be hosts to cationic clusters composed of three or more In atoms. In some cases, these are the sole cationic components, but their very geometry, for example, angular In 35+, tetrahedral In57+, or chain-like In57+ and In68+, clearly implies the presence of indium atoms in different oxidation states. As with similar negatively charged clusters in metal indides, it is by no means easy to ascertain the formal charge, still less the actual charge, carried by these cationic clusters, but they appear generally in their electron count and bonding to conform to the Zintl-Klemm concept and Wade’s rules.26-28 Methods of synthesis are not radically different from those outlined in the preceding subsection, made more elaborate only by the multicomponent nature of several of the products. Elemental indium is a common starting point, and thermal synproportionation, or occasionally oxidation, is the principal means of achieving the desired intermediate oxidation states. Molybdenum reduction of In2O3 and MoO2 affords the molybdate In5Mo18O28,385 while a rare excursion into metal indide territory is made in the oxidation of NaIn by a mixture of Na2O2 and Na2O to form an indate most probably with the composition Na23In5O15.429 Among the first to be structurally characterized were the indium-rich chalcogenide phases In4E3 (E ) Se or Te).383,384 Black crystals of these two compounds are made up of endless interlocking chains composed of five-membered In3E2 rings and are cross-linked by strongly bound [In-In-


In]5+ with In-In spacings of 2.77-2.79 Å and ∠In-In-In ) ca. 158°. In3 clusters appear also to be present in the indium zeolite A, In10Si12Al12O48‚In, prepared by complete redox exchange of indium for thallium in fully dehydrated thallium(I) zeolite A, Tl12Si12Al12O48,430 but here they are assigned a formal charge of only 2+, which is in keeping with much longer In-In distances of 3.073(8) Å [∠In-InIn ) 148.0(9)°]. The crystal structure of what was believed to be the indium-rich sulfide In5S4, formed by heating the elements in the presence of tin, was judged to contain tetrahedral InIn4 clusters, although this necessitated the assignment of a formal charge of 8+, in conflict with the electron accountancy of Zintl’s and Wade’s rules.431 However, subsequent studies including microprobe analysis and variations in the method of preparation have established that the compound is in reality SnIn4S4 and that the clusters in question are not homonuclear In5 but heteronuclear tin-centered SnIn4 tetrahedra for which a formal of charge of +8 holds no contradictions.432 The same unit is to be found in the analogous selenide SnIn4Se4,433 and replacement of tin by germanium is achieved in the corresponding germanium compounds GeIn4E4 (E ) S or Se).391 Homonuclear tetrahedral InIn4 units do appear, however, in the red crystals of the indate NaxIn5O15 for which x is most persuasively assigned a value of 23 consistent with the expected formulation In57+.429 Similar clusters have also been identified in the zeolite In10Si12Al12O48‚In,430 although the formal charge of +8 proposed for each unit must be open to some doubt; these units appear relatively resistant to attack when the zeolite is exposed to an overpressure of H2S.160 On the other hand, In5 clusters of a rather different kind have been characterized in the reduced molybdates In5Mo18O28385 and In11Mo40O62.434 The first is polymorphic with anionic clusters composed of four trans-edge-sharing MoO6 octahedra cross-linked by slightly kinked chain-like [In5]7+ polycations with In-In distances of 2.616 and 2.658 Å that occupy channels within the structure. The second is metallic with complicated magnetic behavior. It is made up of anionic clusters of either four or five trans-edge-sharing MoO6 octahedra, and channels between these units again accommodate short cationic chains, but this time extending to either five or six indium atoms with formal charges of +7 or +8, respectively. The In57+ cations have the same dimensions as in In5Mo18O28, and the terminal atoms of the chain are bent toward three O atoms of the anionic network so that ∠In-In-In ) 158°, implying that these approximate to InII and the intervening atoms to InI. The length of the polycation chains is prescribed, it seems, by the length of the anionic molybdate oligomers. Thus, a Mo4 unit is matched by an In57+ cation and a Mo5 unit by an In68+ one, the terminal atoms of which are also bent toward three O atoms of the anionic network so that ∠In-In-In ) 163°.434 The same unit with the same features, including In-In distances in the range 2.645(9)-2.689(9) Å, is the sole cation in In3Mo11O17, the anionic framework of which is built exclusively on oligomeric clusters of five trans-edge-sharing MoO6 octahedra.435 The metallic conductivity reported for both In11Mo40O62 and In3Mo11O17 depends intimately on the sizes and electronic intercoupling of the different clusters. To the small subset of well authenticated heteronuclear polycations represented so far by [MIn4]8+ (M ) Ge or Sn)391,432,433 has recently been added a more exotic member in octahedral [PtIn6]10+. First identified as one of the cationic

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components of the ternary fluoride PtIn7F13,388 this has subsequently been shown also to occur either as the sole cationic component of the quaternary/quinternary oxometallates PtIn6(GaO4)2-x(InO4)x (0 e x e 1)389 and PtIn6(GaO4)2-x(GeO4)xOx/2 (0 e x e 2),390 or, together with In+, in the quinternary oxide fluoride Pt2In14Ga3O8F15.242 The anionic framework of PtIn7F13 consists of [InIIIF6]3- octahedra and isolated F-, whereas PtIn6(GaO4)2 has a pentlandite structure type in which the [PtIn6]10+ cations are linked Via [GaO4]5- tetrahedra into a three-dimensional network. In PtIn6(GeO4)2O, which is isotypic with the mineral sulfohalite Na6FCl(SO4)2, the building units are [PtIn6]10+, [GeO4]4tetrahedra, and isolated O2- occupying the centers of the In6 octahedra formed by six adjacent PtIn6 octahedra. The oxide fluoride Pt2In14Ga3O8F15 is a new structure type in which the characteristic building blocks are PtIn6 octahedra, GaF6 octahedra, and GaO4 tetrahedra. The Pt-In distances within the [PtIn6]10+ octahedra vary somewhat from compound to compound but fall usually in the range 2.53-2.57 Å; only in Pt2In14Ga3O8F15 does the distance increase to 2.62 Å. They are therefore relatively short compared with the analogous distances involving Pt hexacoordinated by In in intermetallic indides (cf. LaPtIn3 2.69 Å,436 Sr2Pt3In4 2.65 Å,437 and CaPtIn2 2.77-2.79 Å438). Electronic band structure and molecular orbital calculations242 point to a negative rather than a positive oxidation state for the Pt atom in [PtIn6]10+, implying that the original “daring” formulation [Pt-IIInII6]10+388 was not so wide of the mark. On this basis, [PtIn6]10+ is an 18-electron complex. The compounds themselves are diamagnetic and vary from colorless Pt2In14Ga3O8F15 and pale yellow PtIn7F13, both of them insulators, to black, semiconducting PtIn6(GaO4)2. There is an intriguing change in the intense colors displayed by members of the series PtIn6(GaO4)2-x(GeO4)xOx/2 from black at x ) 0 to red at x ) 1, and to yellow at x ) 2.390 DFT calculations suggest that an O atom at the center of an In6 octahedron (which see) cuts the In 5p/In 5p bonding interactions between adjacent [PtIn6]10+ octahedra, thereby raising the bottom of the conduction bands; the resulting quantum dot effect is then responsible for the variations of color. The discovery of [PtIn6]10+ naturally stimulates the inquiry of whether analogous polycations are formed by other heavy transition metals, and reference has certainly been made to the preparation of compounds featuring [IrIn6]9+ octahedra,388 but substantive reports have yet to appear.

5.4. Homonuclear or Heteronuclear Neutral or Anionic Clusters From this category, we exclude not only the anionic clusters of intermetallic indium compounds, but also clusters each containing indium exclusively as InI {as in [(Me3Si)3CIn]4182 and [(η5-C5Me5)In]666,164}, InII {as in [(Me3Si)3CInX]4 (X ) Cl or Br)},198 or InIII {as in the heterocubane molecules [(Me3Si)3CInE]4 (E ) O, S, Se, or Te)200,201}. That leaves a relatively sparsely populated division comprising neutral or anionic molecular species that are built on homonuclear or heteronuclear clusters including In atoms in more than one formal oxidation state. In that the recognized compounds include only three that can strictly be classified as “metalloid” clusters in which a metallic core having only In-In bonds is surrounded by a shell of oxidized In atoms (InI), indium plainly lags well behind both aluminum and gallium.4,6-8 Although the reduced strength of In-In compared with Al-Al and Ga-Ga bonds may have


a part to play in this deficiency, it is much more likely to depend on kinetic than on thermodynamic factors and on technical issues of accessibility. For the synthesis of metalloid clusters of aluminum and gallium has depended on the availability of metastable solutions of the subhalides AlX and GaX in an appropriate solvent mixture (usually toluene and a σ-donor component such as Et2O).4,6-8,285 The disproportionation to the metal and trihalide MX3 (M ) Al or Ga) that occurs when the solutions are warmed, say, from -80 to about +80 °C can be partially frustrated by the introduction of bulky substituents R- {e.g., R ) N(SiMe3)2, SitBu3, or PtBu2} to deliver isolable metalloid cluster compounds MxRy, where x > y. By variation of the reaction conditions (e.g., temperature, solvent mixture, and amount of substituent), it has proved possible to control the degree of disproportionation and hence the size of the metalloid cluster. Such an option has been denied as yet to indium by the failure to find solvent mixtures able to support the indium(I) halides in useful concentrations at temperatures low enough to prevent spontaneous disproportionation to the metal and InII or InIII compounds.259,277,283 As it turns out, disproportionation occurring on thermal decomposition of the corresponding InI or InII compound has been the means of delivery of the metalloid indium cluster compounds identified so far, namely, In8Ar4 (Ar ) C6H32,6-mesityl2),386 In8(SitBu3)6,341 and In12(SitBu3)8382 (see Table 7). Disproportionation, again achieved through serendipity more than design, has also given rise to the heteronuclear clusters featured in [MeC{CH2NSiMe3}3In2]2,140 In9(PPh)4(P2Ph2)3Cl7(PEt3)3,392 (dppe)2Au3In3Cl6(THF)3,281 and [(dppe)2Au]+[(dppe)2Au3In3Br7(THF)]- (dppe ) Ph2PC2H4PPh2);282 it is presumably also responsible for the formation of the anion [{(Me3Si)3C}3In3Br3]-, with an In3Br2 core, which is a byproduct of the reaction between InBr and LiC(SiMe3)3‚ 2THF in toluene.290 An alternative, more controlled strategy has involved mild oxidation of the tetrahedral cluster [RIn]4, where R ) C(SiMe3)3, by 1,2-C2H4Br2, AlI3/I2, or propylene sulfide to form the heteronuclear cluster compounds R4In4Br2,198 R3In3I2,199 or R4In4S,202 respectively. Reduction of the diindium compound In2Trip4 (Trip ) C6H2-2,4,6-iPr3) with lithium powder gives in low yield the tetraindium compound In(InTrip2)3‚3OEt2.340 This, too, comes as a surprise since reduction of the corresponding dialane or digallane under the same conditions gives as the only isolable product the radical anion [Trip2M-MTrip2]•- (M ) Al or Ga) partnered with a solvated lithium cation.439 To these routes, moreover, has now been added the thermal decomposition of an indium(III) hydride, with the discovery that the anionic In5 cluster [In5Br8(quin)4]- (quin ) quinuclidine) is one of the products formed on decomposition of an ether solution of the adduct quin‚InH3 containing LiBr.196 Although the reaction is relatively low-yielding and its mechanism far from clear, its reproducibility lends some hope to the possibility of exploiting more widely the decomposition of indium hydrides to bring about the synthesis of indium clusters of higher nuclearity {cf. Al22Cl20(THF)12440 and Ga24Br22(THF)10441}.

5.4.1. Homonuclear Clusters To our knowledge, no more than five compounds can be accounted currently as containing homonuclear clusters of In atoms, these being, in order of increasing nuclearity, In4Trip6‚3OEt2, [In5Br8(quin)4]-, In8(C6H3-2,6-mesityl2)4, In8(SitBu3)6, and In12(SitBu3)8. Smallest of the clusters is the In4 core of In4Trip6 whose crystal structure as a tris(etherate) proves it to be a tris(indyl)-


indane In(InTrip2)3 with a central In atom bearing three InTrip2 substituents.340 The molecule consists of a trigonal planar InIn3 skeleton with ∠In-In-In ) 120° and In-In bond lengths of 2.696(2) Å, somewhat shorter than those characteristic of diindium compounds of the type R2In-InR2 (2.744-2.938 Å). Each C-In-C plane is tilted at an angle of 31.6° to the InIn3 framework, and oriented at about 77° to the aryl ring. Addition of another In atom gives the In5 cluster central to the [In5Br8(quin)4]- anion isolated as the [H(quin)2]+ salt.196 This is thermally unstable in solution at temperatures exceeding -15 °C, and the solid disproportionates to indium metal and the InIII complex (quin)2InBr3, among other products, when warmed above 5 °C. The anion consists of a central In atom tetrahedrally coordinated by four InIIBr2(quin) fragments (cf. In57+) and is thus analogous not only to the [Al5Br8(THF)4]- anion found in partnership with the [Al5Br6(THF)6]+ cation,442 but also to the neutral gallium complex Ga5Cl7(OEt2)5,193 all being characterized by metal atoms with an average formal oxidation state of +1.4. At 2.747 Å, the average In-In bond distance is slightly longer than that in In(InTrip2)3‚3OEt2, in keeping with the decrease in average oxidation state of the metal from 1.5 to 1.4. With the encapsulation of a single In atom by a shell of three or four oxidized In units, In4Trip6‚3OEt2 and [In5Br8(quin)4]- anticipate indium metalloid clusters, even if they lack the sort of central In-In-bonded core that might be associated with the bulk metal. By contrast, the octaindium cluster compounds In8(C6H3-2,6-mesityl2)4386 and In8(SitBu3)6341 fulfill the stoichiometric requirements of metalloid cluster compounds. The terphenyl derivative forms red crystals and the “supersilyl” one black-green crystals. Solutions of the latter are thermally stable up to 100 °C but photosensitive to visible light, exposure to which results in the deposition of indium metal at room temperature. The crystal structures of both reveal distorted cubane-like arrangements of the eight In atoms (see 46 and 47 in Figure 13). The distortion is much less pronounced for the terphenyl compound where the In8 array approximates to D2d local symmetry, with In-In distances in the range 2.875-2.933 Å and averaging 2.915 Å. The structure may be regarded as consisting of two interpenetrating tetrahedra, one made up of four unsubstituted In atoms and the other of four InC6H32,6-mesityl2 units; alternatively it may be regarded as an In4 tetrahedron with each face capped by an InC6H3-2,6-mesityl2 moiety. The supersilyl derivative reveals an In8 cubane core that has been stretched along a body diagonal, giving In-In distances in the range 2.770-3.303 Å and averaging 3.00 Å; this may be described as an In6(SitBu3)4 assembly having a roughly octahedral In6 core, two opposing faces of which are each capped by an InSitBu3 unit. The metal-ligand distances in both compounds are somewhat shorter than those in similar homoleptic InI and InII-InII-bonded species, a feature variously attributed to reduced steric interaction between the ligands341 and/or changes in the metal orbital hybridization or metal charge.386 The bonding in the terphenyl compound has been rationalized on the basis that each Interphenyl moiety donates two electrons and each unsubstituted In atom one electron to the metal-metal bonding in the In8 core, leaving two valence electrons to be accommodated in a nonbonding orbital that is mostly 5s in character and localized on each unsubstituted In atom. The 12 electrons then available for cluster bonding in this scheme would imply a formal In-In bond order of only 0.5, whereas the average


Figure 13. Molecular structures of the indium cluster compounds In8(η1-C6H3-2,6-mesityl2)4 (46),386 In8(SitBu3)6 (47),341 and In12(SitBu3)8 (48).382 tBu methyl groups omitted for clarity.

In-In distance (2.915 Å) falls at the upper end of the range associated with what appear to be In-In single bonds, and is considerably shorter than the interatomic spacing (3.25, 3.38 Å) in indium metal.25 On the same basis, there are 14 electrons available to the In8 framework of the supersilyl compound In8(SitBu3)6, implying that this should be classified as a doubly capped closo compound, in keeping with the doubly capped octahedral skeleton suggested by the crystal structure.341 Such an interpretation in terms of an octahedral In6(SitBu3)4 substrate whose electron deficiency is relieved by the double capping with InSitBu3 groups finds support in the results of density functional theory calculations. A population analysis indicates that the eight In atoms are approximately neutral,341 a conclusion contrasting with the belief that there is appreciable charge separation between substituted and unsubstituted In atoms in the terphenyl compound.386 Largest of the clusters to be reported so far is the In12 framework of the supersilyl compound In12(SitBu3)8, another unexpected product isolated as black-violet crystals that are thermally stable at temperatures up to 100 °C.382 The crystal structure reveals a closed polyhedral framework of 20 triangular faces, not spherical (cf. B12H122-) but ellipsoidal in form (Figure 13, 48). The eight In atoms at the ends of the ellipsoid each carry one supersilyl group, while the four In atoms in the central belt are unsubstituted. A better appreciation of the In12 polyhedron can perhaps be gained by regarding it as consisting of two distorted In6 octahedra, formed by the hypothetical closo species In6(SitBu3)4; the compound can then be described as a conjuncto indane (cf. the boron hydride B20H1622). It may in fact be formed by

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dimerization of In6(SitBu3)4 formed as an intermediate in the decomposition of In2(SitBu3)4.382 No analogue of this polyhedron is to be found in boron or gallium chemistry; only for the polyaluminum anion [Al12{N(SiMe3)2}8]- has a similar framework been established.443 In both cases, the ligands [i.e., SitBu3 or N(SiMe3)2] play a crucial role in determining the structure of the M12 cluster, which can be recognized, intriguingly, as a section of the fcc structure adopted by aluminum or approximated by indium metal in the bulk.25 The In-In distances in the cluster range, like those in In8(SitBu3)6, from almost 2.80 Å (suggestive of In-In single bonds) to 3.30 Å (comparable with the interatomic spacings in the metal). A space-filling model indicates that the surface of the central In12 cluster is almost completely covered by the eight SitBu3 groups, a factor that probably accounts not only for the relative stability of the compound, but also for the apparent reluctance of the lighter group 13 elements to form analogous species. This, then, is the current extent of metalloid cluster chemistry for indium. Where aluminum and gallium present a developing scene numbering in several tens clusters with a variety of different topologies and nuclearities up to Al77 and Ga84,4,6-8 indium can lay claim to only three such species with a nuclearity not exceeding In12, and no significant advance in the past 5 years. Without some new experimental initiative, it may be that in its beginning is also the end of indium metalloid clusters. There is no obvious reason of chemical principle why indium should not follow where aluminum and gallium have led; in Oscar Wilde’s words, however, “nature has good intentions, of course, but, as Aristotle once said, she cannot carry them out”.

5.4.2. Heteronuclear Clusters This small group of compounds is heterogeneous both in type and in the makeup of the central cluster. In most cases, indium and nonmetal atoms make up this cluster, namely, In3X2 in R3In3I2199 and [R3In3Br3]- [R ) C(SiMe3)3; X ) Br or I];290 In4S in R4In4S [R ) C(SiMe3)3];202 In4Br2 in R4In4Br2 [R ) C(SiMe3)3];198 In2N3C4 in [MeC{CH2NSiMe3}3In2]2;140 and In9P10 in In9(PPh)4(P2Ph2)3Cl7(PEt3)3.392 In just two cases, the cluster is a mixed metal one, namely, In3Au3 in (dppe)2Au3In3Cl6(THF)3281 and [(dppe)2Au3In3Br7(THF)](dppe ) Ph2PC2H4PPh2).282 We exclude of course other clusters, such as Br3In3Co4(CO)15444 and R4In4E4 [R ) C(SiMe3)3; E ) O, S, Se, or Te]200,201 where the indium is in the +3 oxidation state. However, mention should be made of gaseous molecules of the type InnO with n ) 1-8, which have been produced by laser vaporization from an indium target.445 Photoionization followed by mass spectrometric analysis yields ionization potentials for these molecules that have been successfully reproduced, together with the geometric and electronic structures, by appropriate DFT calculations. The diiodotrialkyltriindane R3In3I2 [R ) C(SiMe3)3], formed in good yield by the iodination of [RIn]4 with a mixture of I2 and AlI3, has been isolated as yellow crystals.199 These are thermally quite stable, decomposing with the formation of elemental indium only at 200 °C, although the compound is only marginally stable in solution at normal temperatures. The molecular structure comprises an In3I2 framework that approximates to a trigonal bipyramid in which the equatorial belt is made up of one In and the two I atoms (see Figure 9). Alternatively, it may be described as an angular In-In-In unit [∠In-In-In ) 82.44(1)°], with


each In atom bonded to one alkyl substituent and the terminal In atoms bridged by the two I atoms. The In-In bond lengths (2.814 Å on average) are appreciably shorter than the corresponding distances in the tetrahedral cluster compound [RIn]4 (3.002 Å), being in the normal range for InII-InII single bonds. Despite the different coordination geometries and oxidation states of the three In atoms, the In-C bond lengths are almost indistinguishable (2.221 Å on average), and steric interaction between the bulky alkyl substituents manifests itself in the tilting of the apical In-C bonds toward the iodine bridges. Although prepared subsequently by a quite different route, the analogous gallium compound R3Ga3I2 proves to have a similar structure.446 The same structural motif, in this case an In3Br2 cluster, is found in the [R3In3Br3]- anion, 49, which has been isolated

as its [Li(THF)3]+ salt.290 The anion is the bromide complex of the hitherto unknown neutral molecule R3In3Br2, with the Br- ion coordinated to the central In atom of the In3 chain and linking this chain to the [Li(THF)3]+ counterion. Comparison with R3In3I2 suggests that coordination causes the In-In-In angle to close down [to 77.34(1)°], but has little effect on the In-In and In-C bond lengths (averaging 2.821 and 2.237 Å, respectively). Another pentatomic cluster of trigonal bipyramidal parentage is the In4S cage of the compound R4In4S [R ) C(SiMe3)3], which forms deep red crystals that are stable at temperatures up to 170 °C.202 The framework structure can be described either as a distorted trigonal bipyramid with In and S atoms at the apical sites or as an In4 tetrahedron one face of which has been expanded to be capped by the S atom (see Figure 9). In this case, the compound is clearly recognizable as a thia-closo-pentaindane, the addition of the four-electron donor atom S to the molecular core of [RIn]4 making it electronically isovalent with the hitherto unknown boranes [B5H5]2- and B4H4S, as well as the known 1,5dicarba-closo-pentaborane(5), B3C2H5.447 At 2.838 Å, the average In-In distance to the unique apical In atom lies in the normal range associated with In-In single bonds, whereas the equatorial In‚‚‚In distances are appreciably longer (3.387 Å). The In-S bond lengths (averaging 2.591 Å) are slightly longer than those in the InIII heterocubane [RInS]4 (2.549 Å),200 and the equatorial In-C bonds (averaging 2.227 Å) are slightly shorter than the axial one (2.257 Å), in keeping with the increased formal oxidation state of the equatorial In atoms (+1.67 vs +1). A hexatomic In4Br2 cage is the nucleus of the dibromotetraalkyltetraindane R4In4Br2 [R ) C(SiMe3)3] isolated as orange crystals as one of the products of the reaction between [RIn]4 and the mild brominating agent 1,2-C2H4Br2; by contrast, the action of a Br2/AlBr3 mixture gives the InII cluster [RInBr]4.198 The solid dibromide is stable at temperatures up to 164 °C but, like R3In3I2, it decomposes slowly in solution at room temperature. The molecular structure, as depicted in Figure 9, is unprecedented. It is most aptly described as being derived from an In4 tetrahedron, one triangular face of which is capped by a µ3-bromine atom,


while a second µ2-bromine atom bridges one edge of the same face. The average oxidation state of the In atoms is the same as that in R4In4S, that is, +1.5. The unique In atom at the top of the cluster appears to form more or less normal In-In single bonds measuring 2.842 (×2) and 2.904 Å, but the other In‚‚‚In distances of 3.61 (×2) and 4.07 Å are much longersindeed, are in the range of twice the van der Waals radius of In (3.80 Å).25 Hence there is, at best, only weak interaction between these equatorial atoms. Few though they may be, these products of the partial oxidation of the tetralkyltetraindane(4), [(Me3Si)3CIn]4, do reveal the beginnings of a pattern, as well as pointing to the prospects of further advances. No such pattern or prospects can be readily attributed to the red-brown amide [MeC{CH2NSiMe3}3In2]2, 33,140 and the yellow InP compound In9(PPh)4(P2Ph2)3Cl7(PEt3)3, 50.392 The first is made up of

two In2N3C4 cages and the second of a remarkable In9P10 cage. The two metal atoms in the In2N3C4 cluster of the amide are quite distinct in their siting and formal oxidation state. That occupying the vertex of the nine-atom cage and coordinated to the three N atoms of the tripodal amido ligand bears all the hallmarks of an InII center, being linked to the corresponding atom of the second cluster through an In-In bond measuring 2.807(1) Å; the average InII-N distance is 2.17 Å. By contrast, the second In atom, plainly to be formulated as InI, occupies an exposed, peripheral position bridging two amido N atoms. The 19-atom InP cluster, having an overall diameter of about 0.7 nm, features no less than three InII2 pairs [In(4)-In(5), In(6)-In(7), In(8)-In(9)] with separations averaging 2.741 Å. The other three In atoms [In(1)-In(3)] form no In-In bonds and can be assigned the formal charge 3+. The total charge of +21 is matched if the ligands are considered to be four PPh2-, three P2Ph22-, and seven Cl- ions. If it is assumed that lone pairs are present at P atoms P(4), P(5), and P(6), the In9P10 polyhedron possesses 54 valence electrons for 27 bonds and so features exclusively two-electron, two-center bonds. The cluster core has near-C3 symmetry, the idealized 3-fold axis running through the center of the In3P3 six-membered ring formed by In(1) to In(3) and P(1) to P(3), as well as through atoms Cl(7) and P(10). Altogether the cluster core is built up from four In3P3 six-membered rings, three In3P2 five-


membered rings, and three In2P3 five-membered rings. Of the nine In atoms, three [In(5), In(7), In(9)] are each coordinated by PEt3 groups, while the other six are each bound to one terminal Cl ligand. The seventh chlorine, Cl(7), located at the center of the cluster cage, has to be viewed as Cl-. The isolation and characterization of this compound is no small achievement, but we are tempted to wonder whether, like another celebrated event, it is not a case of “c’est magnifique, mais ce n’est pas la guerre”. Finally, we come to the hexanuclear mixed metal cluster In3Au3 identified by Schmidbaur and his group in the neutral compound (dppe)2Au3In3Cl6(THF)3, 51,281 and the bromide

complex [(dppe)2Au]+[(dppe)2Au3In3Br7(THF)]- (dppe ) Ph2PC2H4PPh2).282 The core is composed of an isosceles triangle of Au atoms, with the two long Au-Au edges (averaging 2.935 Å) spanned by dppe ligands and the short edge (2.562 Å) bridged by an In atom; the Au3 triangle is then capped above and below by In atoms. In other words, the cage can be described as a distorted Au3In2 trigonal bipyramid with apical In atoms and the third In atom bridging one edge of the Au3 equatorial belt. Each In atom is coordinated by two Cl atoms and a THF molecule. The overall geometry of the molecule, together with the shortness of one of the Au-Au bonds, suggests the formal oxidation state 0 for these two Au atoms and +1 for the third Au atom. This then invites the assignment of the formal oxidation states +2 and +1 to the face-capping and edge-bridging In atoms, respectively. All-electron density functional calculations on the model cluster (H3P)4Au3In3Cl6(OH2)3 confirm the formal oxidation states of the Au atoms but find no significant difference of Mulliken charge between the In centers.282 There is no essential change in the cluster core with the switch from (dppe)2Au3In3Cl6(THF)3 to the bromide anion [(dppe)2Au3In3Br7(THF)]-. Apart from the replacement of InCl2 by InBr2 units, the additional Br atom is accommodated as a bridge between a capping and the edge-bridging In atom, thereby replacing the THF molecules coordinated to these atoms in the chloride.282

6. Conclusions Where obserVation is concerned, chance faVors only the prepared mind. LouisPasteur,address,1854 First identified by, and named after, its brilliant indigo blue flame coloration, indium is a rare element with world resources estimated not greatly to exceed 1500 tons.25 Yet it has significant specialist applications, most notably as a component of III-V semiconductor devices, gas sensors, transparent electrically conductive films, and reagents or mediators in organic synthesis.25,93-98,100 Although the chemistry of the element, like that of its lighter congeners, tends to be dominated by the +3 oxidation state, the population

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of compounds featuring the +1 and +2 oxidation states has increased markedly in the past 4 decades. Some of these compounds have physical properties that make them interesting in their own right; others, notably InI derivatives, offer reactivity that makes them potentially attractive as synthetic intermediates. The foregoing account has reviewed indium as it occurs in the oxidation states 0 to +2. In0 species are creatures of the gas phase, which may be trapped in solid noble gas matrices and detected or interrogated by their spectroscopic properties. Hence In atoms in the ground electronic state have been shown to form loosely bound adducts with a number of neutral molecules; calculations on selected 1:1 species indicate binding energies that rise from a few kilojoules per mole for In‚Ar and In‚N2 to about 40 kJ mol-1 for In‚CO.88 Photoactivation of some of these adducts results in oxidation of the metal with insertion into a bond of the coordinated molecule to give monomeric InII molecules such as CH3InH121 and HInNH2.116 Little is known about the weakly bound dimer In2, but photoactivation of the matrix-isolated molecule in the presence of H2 has been shown to give the cyclic InI molecule In(µ-H)2In, which isomerizes to HInInH under the action of light with λ > 450 nm.91 Simple monomeric InI molecules such as InH and InX (X ) F, Cl, Br, or I) are likewise confined to the gas phase or to matrix environments. The aggregation or disproportionation of the InIX species that would otherwise occur in the condensed phases at ambient temperatures can be inhibited by making the substituent X sufficiently bulky, for example, η1-C6H3-2,6-Trip2,71 η5-C5H4PPh2,163 η5-P3C2tBu2,170,171 η2{NDippC(CF3)}2CH,177 or η3-(3-tBuPz)3BH.172,173 It may take only a small decrease in this bulk for oligomerization of the InX moieties to occur Via weak In‚‚‚In bonding, as in [In(C6H3-2,6-Dipp2)]2,161 [InC(SiMe3)3]4,182 and [In(C5Me5)]6.66,164 Stabilization of In+ ions can otherwise be achieved in association with a large anion of low basicity, for example, CF3SO3- 203 and EF6- (E ) P, As, or Sb),260 or by incorporation in a rigid anionic network, as with InX (X ) Cl, Br, or I), InMBr3 (M ) Mg or Cd),225,226 and InMo4O6.239 Indium(I) represents a relatively reactive state of the metal, with a predisposition to disproportionation; apart from reaction channels involving metathesis or oxidation, a relatively rich coordination chemistry, in which the InI state appears to be retained, has started to unfold, as represented by products such as M(InR)4 [M ) Ni or Pt; R ) C(SiMe3)3],72,212 [HB(3,5-Me2Pz)3]InW(CO)5,220 and InSO3CF3‚dibenzo[18]crown-6.203 However, the rate of advance has been significantly impaired by the failure as yet to find a wholly suitable supporting medium for the indium(I) halides, which are the obvious departure points. Paramagnetic, monomeric InII species, such as InH2114 and HInNH2,116 are short-lived transients under normal conditions but can be trapped in noble gas matrices and characterized by their IR spectra. Otherwise they are liable to disproportionate or to dimerize with the formation of an In-In-bonded product. Compounds of the type X2InInX2 with tricoordinated metal atoms survive under normal conditions only in the cases where X is a bulky organic or pseudoorganic substituent, for example, CH(SiMe3)2,339 Trip,340 or SitBu3.52,287,341 Preservation of the In-In bond can otherwise be achieved only by expanding the coordination shells of the metal atoms through the inclusion of neutral or anionic donor species, as in In2I4‚2PnPr3346 and [In2Br6]2-.270,344,345 What approximate to InII2 pairs are also to be found in the


extended structures formed by binary and ternary indium chalcogenides363-371 and the phosphates In3(PO4)2 and In2OPO4.372,373 Even more than indium(I) compounds, derivatives of the InII2 unit are vulnerable to disproportionation, and although examples of addition, metathesis, and oxidation are to be found, a high degree of selectivity is seldom in evidence. Mixed or intermediate valence compounds are each characterized by the presence of indium in two or more different formal oxidation states between 0 and +3. These range widely from systems with discrete metal or dimetal centers, for example, InI[InIIIX4] (X ) Br or I),270,332 [(mesitylene)2InI][InIIIBr4],412,413 Cl3InIII‚InI(dibenzo[18]crown6)Cl,203 and InIInII2InIII2E5X (E ) S or Se; X ) Cl or Br);371 through cationic clusters, for example, In57+,385,429,434 In68+,434,435 and [PtIn6]10+;242,388-390 to neutral or anionic clusters, for example, In8(SitBu3)6,341 [In5Br8(quin)4]-,196 and [(Me3Si)3C]4In4S.202 The first class is now a relatively mature one with many obvious parallels with, as well as subtle differences from, analogous gallium compounds.25 While some may act as a source of InI on dissolution, competition with synproportionation to form InII-InII-bonded species, as well as disproportionation, tends to limit their use in rational synthesis. The second class belongs only to the solid state, there being some evidence to suggest that the size of the polyindium cation can be controlled by appropriate tailoring of the anionic network.385,434,435 The third class, with its inclusion of only a few examples of metalloid clusters, is little more than embryonic. Such unfulfilled promise remains a challenge to the research community. Comparisons are “odorous”, so it has been said. It is reasonable, nevertheless, to enquire: where stands indium in relation to the other elements of group 13? Reference has frequently been made in the past to the “alternation effect” within a periodic group448 arising from the discontinous variations in effective nuclear charge caused by the intervention of the poorly shielding d and f shells of electrons, and amplified for the heavier elements by relativistic effects. On this basis, gallium is seen in some respects as harking back to boron, while indium finds at least as much in common with aluminum as it does with gallium. It is true that both Al and In atoms have lower ionization potentials than does the Ga atom, bringing a greater degree of polarity to their bonds to nonmetal atoms. It is also true that the greater size of the In atom causes it to follow Al in favoring environments with coordination numbers greater than four. Interestingly, too, indium resembles aluminum metal in forming only a single phase under normal conditions, whereas gallium metal has a variety of forms suggestive of less than isotropic delocalization of the valence electrons.25 This difference is probably reflected in the diversity of cage structures now being revealed in metalloid clusters of gallium.4,6-8 On the other hand, compounds of aluminum in formal oxidation states lower than +3 that are stable under ambient conditions anticipate little of the number and variety of such compounds of indium. The properties of the In atom that appear to be critical in shaping this chemistry are (i) its greater size, (ii) the relatively low ionization potentials of its valence electrons, and (iii) the formation of relatively polar bonds to nonmetal atoms that are appreciably weaker than the corresponding bonds formed by either Al or Ga.25 All these factors operate in favor of compounds of indium in oxidation states lower than +3 as against disproportionation to the element and the corresponding indium(III) derivative. At the


same time, the larger size of the metal atom and the weaker bonds it forms are likely to reduce activation barriers to exchange and redox reactions, whether dissociatively or associatively activated. Hence, despite the dearth of reliable evidence, kinetic factors are probably at least as important as thermodynamic ones in shaping this area of indium chemistry.

7. Acknowledgment We acknowledge with thanks the EPSRC for a postdoctoral Research Assistantship (held by J.A.J.P.) included as part of the financial support of the research at Oxford (for A.J.D.).

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