California Association of Chemistry Teachers
Herberl D. Kaesz University of California 10s Angeles
Organometallic Derivatives of the Transition Elements
O n e of the most exciting developments in chemistry in recent years has been the syntheses of the unusual organo-derivatives of transition metals such as ferrocene, (C6HJ2Fe (I). It was discovered that the ,:>;
$ ;
& ,
('I
Fe
Discovered simultmeously in 1951 by two groups of workers Kealy and Psuson (33) from the reaction of cyclopentadienylm~ne8iumbromide and ferric chloride, and by Miller, Tebbath, and Tremaine (58) from the direct reaction of eyclopentadiene with Iran at 300°C.
I
metal was equivalently bonded to five carbon atoms of each of the ligands. Following this revolutionary d i 5 closure there has been a great surge both of synthetic and theoretical activity in the field of organometallic' derivatives of transition elements with the result that of all the atoms known to bond to transition metals, carbon is proving to be the most versatile. This is all the more striking when one considers that until recently, except for certain complexes like the carbonyls or the cyanides, organometallic derivatives of transition elements were all but unknown. The renaissance of interest in them has had a far reaching effect on our ideas of the nature of the chemical bond, and they have also played an important role in industry. Types of Bonds between Transition Metals and Carbon Atoms
A transition metal may be bonded to a carbon atom of a ligand through a single electron-pair bond, or through a link with multiple bond character. I n the former, the bond is formed in the normal way, by pairing two orbitals, one from each atom. The bonding electron pair may he formed from one electron from each atom, or be Lecture presented at the 4th Annual CACT Summer Conference, Asilomar, California, August, 1962. I would like to thank Maryellen Rienecke for preparing all of the illustrations, except for Fig. 2 and Fig. 4, which were reproduced by special permission of tho publishers of the original work in which they have appeared. I In its most generally accepted definition, the term organometallic is reserved for compounds containing metal to carbon bonds, forming a. special class in the broader group of metalorganic compounds. A general review of organometallic compounds both of main group and transition elements will appear (60). shortly in Scientific Amel-ican, see STONE
donated entirely by the ligand into an empty orbital of the metal. Such a bond is also called a o-bond, in the molecular orbital (M.O.) terminology, because the electron pair is localized on the bond axis. A more detailed description of the M.O. and V.B. theories is given by Coulson (12), chapters IV and V, and a comparison of these theories is made in chapter VI. Because these two have been used interchangeably in the field of organometallics thus far, it will he necessary to alternate hetween them in this review. For ligands in which the carbon atom is hybridized other than sp3, an sp2 or s p hybrid is used for the ubond. In addition to this, the unhybridized carbon p orbitals have the proper symmetry and spatial distribution to overlap with unhybridized metal d-orbitals, see Figure 1, as in the carbonyl, cyanide, isocyanide, acetylide, and aryl groups. In M.O. language, the additional overlap is termed a s-bond, in which the ligand makes use of the s * orbitals, see Figure 1, localized above and below the plane of the metal-carbon bond, with a node (zero-electron density) on the bond axis. The bond order in metal-carbon bonds in which double (or s-) bonding is possible, is usually higher than one, hut rarely if ever attains the full value of two. I n the V.B. picture, a fractional bond-order is obtained through resonance between various canonical structures, as for instance in the metal carbonyls: SM-C-:
6+
-
M=C=CI:
I n transition metal complexes, the s-component of the metal-carbon bond plays an important role. In the broadest of terms, it can be described simply as a mechanism for withdrawing electronic charge from the metal orbitals into bonding regions between the ligand and metal. This serves to strengthen the metal-ligand
Figure 1. Two possible reprerentations of multiple bonding between metal ond carbon of suitable ligand.
Volume 40, Number 3, March 1963
/
159
bond, and otherwise adds to the stability of the entire molecule (see below). The alkyl group is not capable of engaging in any significant multiple bonding with the metal, and it is also observed that simple alkyl derivatives are the least stable of all organometallic compounds of the transition elements. The alkyls, (and many aryls and acetylides) are generally susceptible to moisture and oxygen, and are often thermally unstable (some acetylides are explosive). In ligands capable of a-bonding, an electronegative atom such as either oxygen or nitrogen must be bonded to the metalbonded carbon atom, in order to facilitate the withdrawal of charge from the metal back into the ligand (as in the carbonyls, cyanides or isocyanides). This bonding is not as easily accommodated, apparently, on such hydrocarbon groups as the acetylides and the aryls. A metal may be attached simultaneously to several carbon atoms of a ligand, and this too has been subjected to theoretical analj.sis. Two examples might be mentioned here; the bonding of the metal with either ethylene, or a cyclopentadienyl ring. In the former, an M.O. picture has been most widely used, incorporating the familiar concept of electron-pair donation and back-accepting by the ligand. This is the Dewar model (18) first proposed for silver-olefin complexes, and most widely applied to olefin complexes of platinum by Chatt and his co-workers (see Fig. 2). The ligand is described in terms of molecular orbitals, and these are combined with suitable orbitals (or hybrid-orbitals) of the metal. The ligand donates the electron pair of its a M.O. into a suitable empty hybrid orbital of the metal. Excess negative charge on the metal may then flow back into the metal-ligand bonding regions, by overlap of suitable filled orbitals of the metal with the anti-bonding (a*) orbitals of the ligands. The bond between the cyclopentadienyl ring and the metal has also been extensively studied. A V.B. treatment is presented by Pauling (42), (cf. p. 385). There are 513 canonical V.B. structures which contribute to the resonance hybrid of the molecule. An alternative description of the bonding is offered by the M.O. treatment of Craig, et al. (12~). The ligand molecular
Figure 2. Orbitals used and spatial orrmngement of atoms in the .ombination of ethylene with platinum in (CnHslPtC14-iafler Chott and Duncon-
son, 16).
orbitals are shown in Figure 3a. The five 2 p atomic orbitals of the carbon atoms are combined in various ways to give five M.O.'s for the ring. One of these M.O.'s is singly occupied (one of the a orbitals) and it has suitable symmetry and spatial distribution to overlap with ether the d,, or d,, orbital of the metal (when the principal axis of the metallocene molecule is taken as the z-axis, see Fig. 3b). In this picture, then, the bond between the metal and each ring is taken to be composed of an electron-pair, formed by the overlap of the singly occupied u-M.O. of each ring with the suitable singly
Figure 36. Electron-poir bond from overlop of ringly occupied rM.0. of eoch ring with ringly occupied dSror d,, metal orbitals; after Craig, et d,
1954.
occupied d-orbital of the metal (the d,, for one ring, and the d,, for the other). Electron-pair metal-ring bonding is also arrived at in the Moffitt M.O. treatment (39) in which article an elegant and illuminating explanation is given on the use of symmetry arguments to facilitate such an analysis. The M.O.'s of a number of cyclic polyenes which are able to bond to transition metals, have been presented by OrgeL2 Other M.O. treatments of the metallocenes have been published which postulate greater involvement of ring and metal orbitals. For example, the reader can refer to "Carbon-metal Bonding," J. W. Richardson, in chapter I of Zeiss (56),as well as Wilkinson and Cotton (53a, p. 85). To follow such discussions it is necessary to he familiar with the applications of group theory to quantum mechanics, as briefly treated by Moffitt (59), and also explained in such books as Barrow (3). These have found more or less success in accounting for observed magnetic and spectroscopic properties of these molecules. The main advantage of the m.o. method is that it permits quantitative estimation of such properties, but these are beyond the scope of this review. For a fuller account of the relative merits of valence bond, molecular orbital, and ligand field theories, see Liehr (34), in THIS JOURXAL, and also the more recent article by Pauling (42a.) The Sidgwick Effective Atomic Number Rule
In spite of the calculations that have been made so far, there is still a considerable gap between theory and synthetic advances in this field. To present in Figure 30. Symmetries of rnoleculm orbital= derived from otomic p. orbitali
160
/
Journal of Chemical Education
See Fig. 10.3.I, p. 155 of ref. (41a).
some orderly fashion the large body of factual information that is now available, it will be necessary to rely on some further simplified generalizations. The great majority of organometallic derivatives of the transition metals are diamagnetic, and their chemical composition is dictated by the very useful eflective atomic number (EAN) rule. This was formulated by N. V. Sidgwick in the 1920's (48) before the advent of wave mechanics. Like the rule of two (Lewis) or the rule of eight (Langmuir-Kossel) the EAX is hased on the simple premise that elements will undergo chemical transformations in which, by transfer or by sharing of electrons, they tend to achieve the closed shell configuration of the inert gases. For transition metals, these are the eighteen electrons of the n s, n p and (n-l)d valence orbitals. Each transition metal, in its complexes, tries to add a sufficient number of electrons through coordination (including any that it may have lost through ion formation) to attain this configuration. I n modern theoretical treatments (and from the majority of experimental data), it is generally observed that the metal, in its choice of ligand, apparently strives to make profitable use of as many of its valence orbitals as possible. This is held to be,=in principle, no more than a restatement of the simple rule of Sidgwick. Since the material presented below constitutes, in my opinion, some very exciting chemistry, one should strive to introduce some of it at any level at which chemistry is taught. At the early stages students will not have been exposed to a sufficient amount of modern valence theory, but it will be seen that no more sophisticated theoretical framework than the Sidgwick EAN rule is really needed under these circumstances. In that case, those few organometallic complexes that are paramagnetic, and/or in other ways do not obey the rule, may be treated separately. I n essence, the same difficulties hold for these complexes with respect to the EAx as is true for the paramagnetic molecules such as O2or NO, in relation to the octet rule. One word of caution shonld be mentioned in connection with the use or the teaching of the EAK rule. If its underlying principle is presented too literally, i.e., that a metal is trying to "add" electrons to its valence shell, it would grossly misrepresent the true nature of these elements. I t is of course ludicrous to assume that anywhere from six to fifteen electrons may be added to the valence shell of a metal, which is by nature electropositive (electron releasing). I t should he stressed that much of this additional charge is not accommodated on orbitals wholly of the metal but in the internuclear regions between the ligand and metal. As such, they constitute binding forces the better to attach the ligand to the metal. As some organometallic complexes of the transition metals are unusually stable, this is not a meaningless statement. Therefore, if it is emphasized that the electrons being "added" to the metal to conform to the EAx are in reality being shared with the ligand atoms(s), there is nothing about the qualitative ideas of this rule that violate the hasic physics of the bonding in these complexes.
metal through a 0-bond. Although an aryl group, such as the phenyl group (CsH6)possesses the required features for forming metal-carbon bonds with some =-character, the properties found for the known aryl derivatives of transition metals are such that it is obviously more realistic to discuss them along with the u-bonded derivatives (the r-character between the phenyl carbon atom and the metal is not very prominent.). Most of the simple alkyl (or aryl) derivatives, are stable only at reduced temperatures and are also extremely sensitive to air and moisture. Upon warming to room temperature or a little above, they are found to decompose to the free metal and the hydrocarbon radicals. While the discovery of the Grignard reagent (RMgX) in 1900 led to the rapid development of alkyl and aryl derivatives of the main group metals, the treatment of transition metal salts with this reagent led mostly to reduction of the metal and coupling of the hydrocarbon group. The coupling reaction is of interest in its own right (See Cotton (If), p. 551). (It was the attempted coupling of two cyclopentadienyl rings which led to the unexpected discovery of ferrocene!) The only really stable simple alkyls of transition metals are the methyl derivatives of platinum (11) to (IV). Tetramethylplatinum (111) exists as a tetramer, as determined by X-ray diffraction in 1947 PtCL
+ CH,MgI
-
[(CH,),PtIb rnp 215'
K in benrene
IV n = 2; solution n = 12 or more; crystal
I11
(Pope and Peachey, 1909 ( 4 4 ) )
Lichtenwalter, 1938 ( 8 2 ) )
(47), see Figure 4. The unusual feature of this molecule is that a methyl group is fuilctioning as a bridging ligand, similar to that found for trimethylaluminun~ dimer. In the mixed alkylplatinum halide, (CH& PtI, (11), the halogens are observed in the hridging positions, but this is not suprising as these atoms possess lone pairs which may donate to another metal after a normal covalent bond has been formed to the first metal (cf. AI2Cl6). The bonding in tetramethylplat-
Highlights of Synthetic Developments
a-bonded Alkyl (or oryl) Derivofives Alkyl groups are taken to be bonded to thc transition
Figure 4. Spatial arrangement of otomr in trimethylplotinurn chloride ond tetramethylplotinurn, after Rundle and Sturdivant. 1947.
Volume 40, Number 3, Morch 1963 / 161
inum tetramer is considered electron deficient. A recent review of this topic has been presented by Stone (49). The tendency of platinum to engage more than four groups in honding is observed even at the expense of creating extremely unusual circumstance for a methyl group. It may readily he seen that with ten electrons in the 5d and 6s orbitals, platinum (in the "zero-ualenl" state) lacks eight electrons to attain the electronic structure of xenon. Covalent bonding to four methyl groups add only four more to the valence shell, leaving the metal "unsaturated." It would then seek additional sources of electron density, which is available through the formation of methyl bridges. Very few simple alkyl derivatives have been formed for other transition metals."n these derivatives the metal is often coordinatively unsaturated, and they often strongly retain Lewis base solvent molecules (such as tetrahydrofuran, THF, as in (CsH&Cr.(THF)3, (V). When the solvent is pumped off, even a t low temperature, complete breakdown of the alkyl (or aryl) to metal bonds occurs. These derivatives also tend to add additional carbanion groups (R- or Ar-) to form anionic complexes. These are isolated as double salts, such as LiMn(CH& or Li3Cr(CsH&.2..5EtzO. Some simple alkyl or aryl derivatives have been reported for Ti, Cr, Mn, Pt, Cu, Ag, Au. Those that are known almost always d o not satisfy the EAN rule for the metal. Often they are paramagnetic, which may be taken in a rudimentary way, as evidence that not all the degenerate d orbitals of the metal have been engaged in bonding. The fact that the EAN rule is not obeyed relates to their general instability, as discussed below. On the other hand, because of this, these compounds are exceedingly useful reaction intermediates. It will be sufficient to mention one or two examples. I n the first place, hydrolysis of the tetrahydrofuran complex of triphenylchromium, (V), leads to interesting ?r-complexes (VI and VII) in which the metal is equivalently bonded to six carbon atoms of the ligand. These proved to be the compounds that were originally thought to be the phenyl derivatives of chromium (58),see also the synthesis of dibenzene chromium (21).
VII
The complex, (V), is also active in the polymerization of acetylene. Some of the possible intermediates and some of the products obtained, are presented in Figure 5. This is similar to the activity of the complexes a For a comprehensive review, see "Transition Metal AIkyls and Aryls," G. E. COATES ANDF. GLDCKLING, chap. 9, ZEISS(56').
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Journal of Chemicol Education
THF
CsH5 R-C=C-R/
I
Figure 5. Porrible intermediates and i d a b l e products in the polymerization of acetylenes by triphenylchromium trirtetrohydrofuranok, after ZeitsI561.
THF
162
of nickel, which are generally referred to as Reppe catal y s t ~ . This ~ general field is of great interest to the petroleum industry where low molecular weight byproducts of the refining process, such as ethylene, propyleue, and acetylene are rendered more valuable when converted into polymeric materials. The instability of simple alkyl (and aryl) derivatives of transition metals has received theoretical interpretation through a quantum mechanical calculation by Jaffe and Doak (SO). These authors believe that the normal sigma bonds from sp3 orhital of carbon atom to suitable orbital on a transition metal are weaker than those to main group metals because they have (1) approximately only a third of the ionic resonance energy which makes for stable bonds to alkali and alkaline earth metals and (2) about half the covalent energy (involving terms of overlap integral and ionization potential of the bonded atoms) that make for stable bonds to all the other main group elements. The aryls of transition metals are somewhat more stable than the corresponding alkyls; the honding orbital of the carbon of the phenyl group contains more s-character (sp?, 331/37&-) compared to that in the alkyl group (sp3, 25'%s) and this will aflect in a favorable direction the ionization potential, overlap integral, and the ionic resonance energy of the carbon metal a-bond. A carbon sp2 orbital is more electronegative than sp3 (See Coulson (I$), p. 218).
In the period following the Jaffe and Doak article, a host of derivatives containing stable alkyl and aryl groups a-bonded to transition metals have been synthesized such as (VIII) through (XIII). The outstanding feature of these is that due to the presence of other ligands in addition to the a-bonded groups, the metal appears in the final compound to have attained the electronic structure of the next inertgas. (The a-bonded alkyl groups count as one, the carbonyls or phosphines as two, and the neutral cyclopentadieoyl ring as five electron donors, all counting the metal in the "zerovalent" state, see below.) I t would appear that the transition metal must be "conditioned"~to form stable metal-to-carbon a-bonds. 'See "Arene Complexes of Transition Metals," by H. Zeiss, ANT) EICI~LER (47a) and chap. 8, ZEISS(,56); also, SCHRAUZER ZEISS( 6 7 ) . =See CHATTAND SHAW(7), or the review by COATESA N D CLOCKLING, chap. 9 in Z ~ r s s(56').
This conditioning often consists of surrounding the metal atom with sufficient ligands for it to achieve its EAN. From the theoretical standpoint, these additional ligands would be expected to alter the nature of the metal orbital(s) engaged in a-bonding from those which are used in the simple akyl or aryl derivative. In M.O. language, the effect of these ligands would be essentially to increase the energy difference between the bonding orbitals, on the one hand, and the non-bonding and anti-bonding orbitals on the other. This would make the latter regions of space less accessible to the valence electrons. The gain in stability of the complex whencver the metal has attained its EAN can also be interpreted through a kinetic effect. Un-interacted low lying orbitals (either empty or partially filled) would tend to make the derivative in which these are found more susceptible to chemical attack. Such a species would be more easily converted to any other accessible set of products thermodynamically more stable. On the other hand, complexes in which all the low-lying orbitals have been engaged in bonding would present a significant barrier to further chemical conversion, their stability being determined kinetically rather than thermodynami~ally.~The operation of a kinetic-stabiliza6 See also the emlanation for the resistance to hvdrolvsis of 5 4 , compared to S'eFs or TeFs, as can he found in Gats such as COULD (ZS), p. 300. ~~~
tion may be illustrated by the evidence for the unusual benzylpentaquochromium ion, CsHGH2Cr(OH2)s.++ This is found to have a half-life of 1.5 days in aqueous solution, in the absence of air ( I ) . It is formed in the reduction of benzyl chloride by chromous perchlorate in dilute perchloric acid solution, and owes its stability to the non-labile character of chromic complexes. I n general, it is not possible to surround a transition metal with a sufficient number of alkyl or aryl groups to achieve the stabilizing effect obtained by use of other types of ligands. In the fmt place, the a-bonded groups are only one-electron donors (considering the metal in the "zero-valent" state). Since the transition metals lack anywhere from eight to fifteen electrons to attain their EAN, it would in most cases be stericallj impossible to achieve this goal solely with u-bonded groups. In addition the failure of the a-bonded groups to engage in =-bonding, i.e., to relieve the metal of negative charge, also contributes to the instability of such derivatives. Two important recent developments involving a-bonded alkyl derivatives should be mentioned. Within the last two years, perfluoroalkyl derivatives of transition metals have been synthesized, (8) and they appear to be more "stable" than their hydrocarbon analogues.' Often, fluorocarbon derivatives are knomn where the hydrocarbon analogues have not been available i.e., XIV, XV, and XVI. In the perfluorocarbon
VIII XIV
:
XVI
RCo(CO), XIb
group, the strong electron withdrawing properties of the F-atoms acting through the C-F bonds serve to increase the electronegativity of the carbon atom bonded to the metal, which in turn will affect the "stahilityJ'of the derivative in question (see above). Lastly, the unusual reactivity of the alkyl-metal carbonyls, particularly those of cobalt, deserve special
' The term "stability" as used here, refers mainly to qualitative chemical observations, such as thermal stability, or resistance to oxygen and moisture. Such usage is known t o offend tht sensibilities of certain chemists, and perhaps the term "chemical robustness" should be used instead, as suggested recently by Prof. G. E. C 0 . 4 ~ ~(during 8 dirrcussions arising at the Symposium on Organometdlic Compounds, University of British Columbia, Vancouver, B. C., Sept. 1962). Volume 40, Number 3, March 1963
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163
mention. The carbonyls and the hydrocarbonyls of cobalt have long been known to display catalytic properties in two well known organic reactions, the "0x0" (hydroformylation) reaction, (XVII), and the FischerTropsch synthesis, (XVIII). Both of these involve two novel reactions of alkylcobalt carbonyls. I n the first of these, a metal alkyl derivative is formed by Co.(CO)a
+ CO + Ha RCHS-CHI--CHO Ca(CO)a C,Hn.+a + n H20 n CO + (2n + 1) Ha
RCH=CH2
L
XVII
II
+
-
RCC~(CO)I HX
0
I/
RCH
+ HCo(CO)*
XXI
XIX
XXIII
XXIV
XXV
XXVII
0
1
(il co (ii) 2Ha
(repeated many times)
R(CHd.Co(COj4
Olefin Complexes
It is very likely that the first organometallic derivative of a transition element was the olefin complex discovered in 1827 known as Zeise's salt (55), [CHF CH2.PtCI3]-, K+.2H20. Since that time, olefin complexes of other transition metals have been discovered, such as the silver-olefin complexes, and in 1930, the bntadiene adduct of iron carbonyl, C4Hs Fe(CO)a, Reihlen, Gruhl, Hessling, and Pfrengle (46). A comprehensive review of olefin compounds has been given by Fischer (20) or Zeiss (56). After the discovesy of ferrocene in the early 19.50's, complexes of olefins and transition metals have been synthesized in great profusion. It is now clear that a transition metal can be attached to an olefin by engaging simultaneously anywhere from two to seven of its carbon atoms in bonding. A sample of some of the known complexes is given in structures (XXI) through (XXX), as well as in some of the other structures presented in this article. To determine whether the metal has achieved its EAX in any of these, any one of a numher of alternative schemes may he followed. In the neutral complexes, it is convenient to consider the metal in the "zero-valent" state, and the ligand as a neutral olefin or radical. A ligand attached to a metal through n carbon atoms may be considered an n-electron donor; one singly occupied p orbital from each olefinic carbon 164
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Journal of Chernicol Education
c coo
0 XXII
XVIII
addition of metal-hydrogen of a hydrocarbonyl across the carbon-carbon double bond of an olefin, such as reaction (XII), above. Under pressure (1000 to 6000 p.s.i.) and about 125'C, carbon monoxide may then be inserted between the alkyl group and the metal, such as shown in the reversible reactions (XIb) and (XIa), above. Depending on the conditions, two routes are possible from this point. I n the 0x0-process, hydrogenolysis of the acyl group from the metal gives the aldehyde, (XIX). On the other hand, in the Fischer-Tropsch synthesis, the acyl group is reduced to a new alkyl group containing an additional CH2 unit, (XX), which can in turn repeat the process (XIb) to (XIa) and with alternate reduction and carbonylation, lead to higher molecular weight alkanes. 0
C
0
XXIX
XXVIII
Q & XXX
atom is involved with the metal. I t is equally acceph able to regard certain ligands as negatively charged. which must then be balanced by suitable positive charre on the metal, if the complex is neutral. For instance, in the cyclopentadienyl derivatives, a neutral CsH6 ring is regarded as a 5-electron donor to a "zerovalent" metal, or as a negatively charged ring, CsH6-, donating 6 electrons into a suitably ionized metal atom. Application of the EAN is only a formalism and it can not be stated with certainty which of these two alternative approaches is the more correct one. Most olefin-metal complexes are diamagnetic and obey the EAN rule. The chemistry of the cyclopentadienyl derivatives has recently been reviewed in THIS JOURNAL (46). The Carbonyls ond Mixed Olefin-Mefol Corbonyl Complexes
The carbonyls have been recognized as an unusual type of transition metal complex since the discovery of Ki(CO)*by Mond, Langer, and Qnincke in 1890 (40). They obey the EAK rule with the sole exception of V(CO)e, which is monomeric and paramagnetic (one unpaired electroi~).~Recently, some substituted carhonyls have been isolated in which the E A S rule is not oheyed, for instance, some phosphine and diarsine a The chemistry of the carbonyls has recently been reviewed is THIS JOURNAL, see PODALL (45). For a comprehensive treatment see CHATT,PAUSON A N D VENANZI, chap. 10 in ZEISS (66).
complexes of manganese carbonyl, ( C B H ~PMn(COjr )~ and [ C ~ H ~ ( A S R & ] % ~ ~ ( CNo O ) simple ~. explanation is yet available why these and some other similar complexes do not exist as dimers, with metal-metal bonds, as does, for instance the parent carbonyl, Mnp(CO)lo. Last year, the preparation of TcI(CO)~o,long missing last member of the manganese sub-group carbonyls, was reported (27a, 276). Technetium does not occur in nature and is available only through synthetic nuclear processes. It is now being recovered in gram quantities from residues of atomic reactors, and we should be s e e ing more of its organometallic chemistry reported in the next few years (see below). Two types of metal-carbon bonding are generally recognized in the carbonyls; the terminal CO group, in which the carbon atom is bonded to one metal, and the bridging group in which the carbon is attached to two metals. I n the former, the ligand is taken to donate two electrons to the metal; in the latter, one electron is being donated to each of the two metals. The manner with which these two types of bonding, and also the formation of metal-metal bonds are combined in the known carhonyls to give the EAN for the metal is amply discussed in the references cited above,8 and also in many standard texts. The carbonyls often serve as convenient starting materials for the synthesis of olefin complexes. Many instances are known where mixed olefin-carbonyl derivatives are isolated. In olefins attached to the metal through an even numher of carbon atoms, each double bond can displace a carbonyl group, such as in the complex (XXI), above, which may be considered a substitution product of Mn(CO)6+,A1C14-. I n the dienewmplexes, such as (XXVIII) and (XXV) above, two CO groups in the starting material (Fe(CO)6) have been displaced from each metal atom. I n benzene-chromium tricarbonyl, (XXXI), the olefin has replaced three
Cr
such as in allylmanganese tetracarbonyl, (XXII), or in cyclopentadienyl manganese tricarbonyl (XXXII). I n the vanadium complexes, (XXXIII) and (XXXIV), the odd electron of the ligand has paired with the odd electron in the starting carbonyl. For other carbonyls in which no metal-metal bonds were present, the odd electron-donating olefin will displace an even number of CO groups, and then the final complex will contain a metal-metal bond, see (XXXV) and (XXXVI). Alternately, the odd electron in such mixed olefin carbonyl complexes may be used for a-bonding to an alkyl (or aryl) group, such as in (IX), above. KO group of ligands has proved itself more versatile than the acetylenes, and their reactions with metal carbonyls deserves special mention. Similar to the activity of the alkyl-transition-metal derivatives (see Fig. 5 above), the carbonyls have also been found to polymerize acetylene, and here also it was principally through the pioneering work of Reppe and his collahorators. Many new and interesting olefins have been synthesized, containing two, three, or four acetylene units. Often, carbon monoxide is polymerized together with acetylene, and unsaturated cyclic ketones and diketones have been obtained. Some of these may be isolated while still attached to the metal carbonyls, (XXXVII) and (XXXVIII). In rarer cases, the acetylene is found bonded to the metal without having nndergone any polymerization, (XXXIX) and (XL). I n the
XXXVII
XXXVIII
XXXIX
Mn
oc" cc' o 0
XXXI
XXXII
XXXIII
XXXIV
XXXV
XXXVI
CO groups in Cr (CO)s. Olefins in which an odd number of carbon atoms are bonded to the metal are coosidered as odd-electron donors. The odd electron must somehow be paired in the final complex, if it is to be diamagnetic and follow the EAN rulc, which the great majority do. The odd electron may take the place of the metal-metal bond (if any) in the starting carbonyl,
latter, this ligand is demonstrating its ability to T-cornplex simultaneously to two metals, owing to the two rr-bonds a t right angles to each other from which the triple bond is composed (see also below). It will be necessary to refer to further sources for more detailed coverage of this field, which by now has grown to vast proportion^,^ hut it would not be possible to leave this topic withont giving a sample of some of the truly novcl structures than have been thus far obtained (see (XLI) through (XLIV)). The adduct Fel(COj6.(RC1R)?, XLI, is isolated from the reaction of acetylene Volume 40, Number 3, March 1963
/
165
with iron carbony1;J hut has also been obtained in the reaction of thiophene with iron carhonyl (31). The cobalt adduct C O ~ ( C O(RC2H)H, )~. (XLII), contains a carbon atom bonded to three cobalt atoms. It can be formed only from mono-substituted acetylenes, as migration of acetylenic hydrogen most likely is involved in its formation (5). It is obtained by treatment of the adduct of the type Coz(CO)s,(RC2H), (XL), with alcoholic HC1 (36). The mechanism of formation of many of these interesting complexes awaits future studies. It would appear that even greater degrees of complexity are possible, with the two structures (XLIII) and (XLIV), however, some degree of order is beginning to be apparent. I n the complex, Cod(CO),O.(RC2R), (XLIII), the arrangement of cobalt
XLI
XLII
oc
e
\I/
XLIV
atoms around the acetylenic carbon atoms is found (15) to resemble somewhat the arrangement found in thorium dicarhide, in which each Ca group interacts with six thorium atoms. I t is likely then that the even more perplexing structure, (XLIV), might also prove to be analogous to other known types of carbides. The function of the lone carbon atom in Fes(CO)& (4), would surely resemble those carbides in which discreet carbon atoms can be recognized (see below). In any case, these complexes are diamagnetic, and it is left to the reader to puzzle whether the metal has achieved its EAN. From these very recent advances, perhaps future work might be predicted. Would it not be of interest to see whether metal carbides can be treated with olefins and/or carbon monoxide, to see whether any other intermediate types of compounds such as (XLIII) and (XLIV) can he obtained? Cyanides, Isocyanides, Acetylides, and Carbides
contain metal-carbon bonds. It is interesting to compare these to the metal-carbon bonds in the alkyl, aryl, carbonyl derivatives and the olefin complexes, as discussed above. No more will be said here, however, than to give leading references where further detailed and up to date information may be found on these. A recent review of the cyanide complexes of transition metals is available, Griffith (26). The CNion is isoelectronic with the CO group. It is also chemically related to the isocyanides through an alkylation reaction, of which typical examples may be: Fe(CN)s4-
+ C.H.1-
+
-
( C I H ~ N = C ) F ~ ( C N ) F I-
F e ( C N ) F + (CHa)~S04 (CH3N=C)6Fe++
+ 3 SO4-
XLV XLVI
Isocyanide complexes of metals have recently been reviewed by Malatesta (35). Complex acetylides of transition metals, containing the ligand R e C - , where R=alkyl, aryl or H, have been reviewed by Nast (41). The acetylides containing the group (C%C)-- are classed with the carbides, of which two main types are recognized; those containing the aforementioned group, and those containing discreet C atoms (or C4- ions). The former yield acetylene upon hydrolysis while compounds of the latter group generally yield methane, see Wells (52), p. 760. Perhaps work in the near future will show a closer resemblance of this type of compound to certain metal olefin and/or carbonyl complexes (see above). Future Trends
Certain areas of organometallic chemistry of the transition elements are probably going to receive widespread attention in the next few years. Certainly, we can expect more developments in the novel fluorocarbon complexes, only recently discovered (see above). Also the newly discovered relationship between the metal carbides and certain derivatives obtained from the reaction of acetylenes with metal carbonyls (see ahove) will very likely be investigated further. It will be exciting to follow these developments, and see how they enhance or modify our ideas concerning bonding of carbon and transition metals. Many other developments in the current literature are so novel that they have not been represented in the typical reactions discussed above. Since the field of organometallic derivatives of the transition elements is undergoing rapid expansiou and change, a selected number of such developments are separately mentioned here. It is known that olefin complexes can be protonated. While in some cases the proton may become attached to the ligand, as in (XLVII) (29),in some other complexes, it is almost certainly attached to the metal, (XLVIII) and (XLIV) (53). The interesting Lewis-base properties displayed by the metal in the latter t y o have been theoretically interpreted (2). I t is apparent
These compounds are included in the general topic of organo-derivatives of transition metals because they 9 See COATEB AND GLOCKLING in ZEISS (66), or COATES(9). The developments in this field are so rapid, it is also necessary to read the current literature, principally under the names: W. Hubel, I. Wender, or'E. R. H. Jones, and their collaborators. For X-ray diffraction structure studies of novel complexes, see under L. F. D:thl, 0.S. Mills, or G. S. D. King, and collaborators.
166
/
Jaurnd of Chemicol Education
hutadienciron tricarhonyl
+ HCI
-
I
XLVII
bond, were the cyclopentadienyl tricarbonyls of the chromium sub-group metals (54), see (XXXV) above, and the carbonyls of manganese and rhenium (IS). Many more compounds have been discovered since, with even more novel metal-metal bonding, including the derivatives (LVII) (lo), (LVIII) (Id), and the recently reported cyclopentadienyl of technetium [(CrH5)2TcI2($8). The metal-metal bond in these accounts for their diamagnetism, and also, these obey the EAN rule. that the non-bonding electrons of the metal in these metallocenes are available for interaction with additional groups. This could be taken as experimental evidence that not all the metal electrons are engaged in bonding to the C5-rings, which lends physical reality to the picture of electron pair bonding between metal and ring of Craig, et al., or of Moffitt (see above). Cationic metal complexes may be reduced by hydride ion, and also in this case, it is possible to observe finalattachment of the entering group either on the ligand, (L) ($41, and (LI) (16), or on themetal, (LII) (17). The reaction may be reversed by trityl fluorohorate, as shown. Certain olefin complexes in which the EAN for the metal is being exceeded will readily undergo oxidation
LVII
or other reactions in which less involvement of electrons (and/or ligand atoms) with the metal are observed. The action of CCI, on cobaltocene (32) and ($5) serves to illustrate this principle, (LIII). Some of the reactions of nickelocene also follow this principle, such as (LIV) (S7), (LV) (I9), and (LVI) (51). The displacement of an entire CSH6group in the latter reaction is closely related to the action of CO to give rfCcHs~NiCO1~. ", - - - - , Lastly, it should be mentioned that we are becoming more aware of metal-metal bonds in organometallic (and other) complexes of transition metals, see compounds (XLI) through (XLIV), as well as some previous derivatives cited in this article. Until about five years ago, it was believed that bonds between transition metals without other bn'dging groups, would not be stable enough to exist in isolatable compounds. The first to be discovered containing a free metal-metal
.\."
LVIII Literature Cited
(1) ANET,F. A. L., AND LEBLANC, E., J. Am. Chem. Soe., 79, 2649 (1957). ( 2 ) BALLHAUSEN, C. J., AND DAHL,J. P., A d a Chem. Scand., 15, 1333 (1961). (3) BARROW,G. M., "Introduction to Molecular Spectroscopy," McGraw-Hill, New York, 1962. 14) , . BRAYE. E. H.. DAHL.L. F.. HUBEL. W.. A N D D . L. WAMPLER, J: Am. Chem. SO;., 84, 4633 (1862). (5) BRAYE,E. H., HOOGZAND, C., HUBEL,W., KRUERKE, U., MERBNYI,R.,AND WEISS, E.,"Advances in the ChemVolume 40, Number 3, March 1963
/
167
istry of Coordination Compounds," STANLEY KIRSCHNER, ed., Macmillan, N. Y., 1961, p. 190. (6) CHATT,J., AND DUNCANSON, L. A., J. Chem. Sac., 1953, 2939. (7) CRATT,J., A N D SHAW,B. L., J. Chem. Soe., 1960, 1718. (8) C & E Neuls, October 2, 1961, p. 52. (9) COATES, G. E., "Organometallic Compounds," 2nd edition, John Wiley and Sons, Inc., New York, 1960. (101 COREY,E. R., AND DAHL,L. F., Inorg. Chem., 1, 521 119RZ1~ - - -- ,. (11) COTTON, F. A,, Chem. Reus., 55, 551 (1955). (12) COUL~UN, C. A,, "Valence," 2nd. ed., Oxford Univ. Press, 1961. (12a) C u m , D. P., MACCOLL, A,, NYROLM, R. S., ORGEL, L. E., AND SUTTON,L. E., J. Chem. Sac., 1954,332. (13) D A ~ LL., F., ISHISHI, E., AND RUNDLE,R. E.. J. Chem. Phm.. 26.1750 (19.571. , (14) DAHL,L. F., MARTELL, C., AND WAMPLER, D. S., J . Am. Chem. Soc., 83, 1761 (1961). (15) D A ~ LL., F., AND SMITH,D. L., J. Am. C h a . Soc., 84, 2451 (1962). (16) DAUREN, H. J., JR., A N D BARTELLI, D. J., J. Am. Chem. Soc., 83, 497 (1961). (171 G.. J. . . DAVISON.A,. GREEN.M. L. H.. AND WILKINSON. Chem. ' ~ o e :1961, , 3172. (18) DEWAR, M. J. S., Bull. Soe. ehim. France, 18, C 79 (1951). (19) DUBECK, M., J . Am. Chem. Soe., 82,6193 (1960). (20) FIscnER, E. O., Prweedings of the Interuati.tiona1Conf. on Coord. Chem., Special Publ. No. 13, The Chemical Socioty, London, 1959, p. 73. (21) FISCHER,E. 0. AND HAFNER,W., 2. 1'Vatu~jor8ch+, lob, 665 (1955). (22) GILMAN, H., AND LICRTENWALTER, M., J . Am. C h a . Soc., 60, 3085 (1938). (23) GOULD,E. S., "Inorganic Rertctions and Structure," revised ed., Holt, Rinehsst and Winston, N. Y. (1962). (24) GREEN,M. L. H., AND NAGY,P. L. I., J . Am. Chem. Soe., 84, 1310 (1962). (25) GREEN,M. L. H., PRATT,L., AND WILKINSON, G., J . Chem. Soc., 1959, 3753. (26) GRIFFITH,W. P., Quart. Kevs., 16, 188 (1962). (27a) HIEBER, W., AXD HERGET,C., Angew. Chem., 73, 579 (1961). (27b) HILEMAN, J. C., HUGGINS, D. K., AND KAESZ,H. D., J . Am. Chem. Sac., 83, 2954 (1961); subsequently also (27a). reported by HIEBERand HERGET (28) HUGGINS, D. K., AND KAESZ,H. D., 3. Am. Chem. Soc., 83,4474 (1961). (29) IMPASTATO, F. J., AND IHRMAN, K. G., J. Am. Chem. Soc., 83,3726 (1961). (30) JAFFE, H. H., AND DOAK,G. O., J. Chem. Phys., 21, 196 \
-".
.
.
. .
(lQXZ\. ~-...,~
(31) KAESZ,H. D., KING, R. B., MANUEL, T. A,, NICROLS, L. D., AND STONE,F. G. A., J . Am. Chem. Soe., 82, 4749 (1960). (32) KATZ,S., WEIHER,J. F., A N D VOIUT,A. F., J. Am. Chem. Sac., 80,6459 (1958). (33) KEALY,T. J., A N D PAUSON,P. L., Nature, 168, 1039 (1951). (34) LIEHR,A. D., J. Chem. Ed., 39,135 (1962). (35) MALATBSTA, L., "Progres8 in Inorganic Chemistry," Vol. 1, F. A. COTTON, ed., Interscienee Publishers, Inc., New York, 1959, p. 283. (36) MARKBY, R., WENDER, I., FRIEDEL,R. A., COTTON, F. A,, AND STERNBERG, H. W., J . Am. Chem. So:., 80, 6529 (1958). (37) McBnmE, D. W., PRUETT,H. L., PITCHER, E., A N D STONE, F. G. A.. J. Am. Chem. Soc.. 84.497 (1962).
(40) MOND,L., LANGER, ( 1890, 57, 749. (41) NAST, R., Intelwtional Conf. on Coord. Chem., Special Publication No. 13. The Chemical Society. London,
168 / Journal of Chemical Education
(41a) ORGEL,L. E., "An Introduction to Tramition Metd
Chemistry: Ligand Field Theory," John Wiley and Sons, N. Y., 1960. L., "The Nature of the Chemical Bond," 3rd (42) PAWLING, ed., Cornell U. Press, Ithaca, 1960, p. 385. (4%) PAWLING, L., J. CHEM.EDUC.,39,461 (1962). (43) PODALL, H. E., J. CHEM.EDUC.,38,187 (1961). (44) POPE, W. J., A N D PEACREY, 8. J., Prac. Chem. Sac., 23, 86 (1907); J . Chem. Soe., 1909,571. (45) Rauscn, M. D., J. CHEM.ED., 37,568 (1960). (46) REIHLEN, H., GRUEL,A., V. HESSLING, G., A N D PFRENGLE, O., Annalen, 482, 161 (1930). (47) RUNDLE, R. E., A N D STURDIVANT, J. H., J. Am. Chem. Soc., 69,1561 (1947). (47e) SCHRAUZER,G. N., A N D EICBLER,S., Chem. Ber., 95, 550 (1962). (48) SIDGWICK, N. V., "The Electronic Theory of Valency," Oxford U.Press, London, 1922; p. 163. (49) STONE,F.G . A.,Endeauour, 20,61 (1961). (50) STONE, F. G. A,, "Organometallic Compounds," Srienlific American, in press (1963). (51) TILNEY-BABSETT, J. F., J . Chem. Soc., 1961,577. (52) WELLS, A. F., "Structural Inorganic Chemistry," 3rd. ed.. Oxford U. Press. London. 1962. (531 G.. " ~ d v m c ein ~ the Chemistrv of Co~, WILKINBON.
. .
ordination Compounds," STANLEYKIRSCHNER,ed. Macmillan, N. Y., 1961, p. 50. (53a) WILKINSON, G., A N D COTTON, F. A,, "Progress in Inorganic Chemistry," Vol. 1, F. A. COTTON, ed., Interscience Publishers, Inc., New York, 1959. (54) WILSON,F. C., AND SHOEMAKER, D. P., J. Chem. Phys.,
27, 809 (1957). (55) ZEIGE,W. C., Pogg. Annalen, 9, 632 (1827). (56) ZEIBS,H., "Organometdlic Chemistry," A. C. S. Monagraph 6147, Reinhold Publishing Corp., New York, 1960. (57) ZEISS, H., "Advances in the Chemistry of Coord. Compounds," S. Kirschner, Ed., Macmillm, N. Y., 1961, p. 88. (58) ZEISS, H., AND HERWIG,W., J. Am. Chem. Soe., 78, 5959 (1956).
+
