and Third-Row Transition Metals - American Chemical Society

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Six-Vertex Hydrogen-Rich Cp2M2B4H8 Dimetallaboranes of the Second- and Third-Row Transition Metals: Effects of Skeletal Electron Count on Preferred Polyhedra Adrian M. V. Brânzanic,† Alexandru Lupan,*,† and R. Bruce King*,‡ †

Department of Chemistry, Faculty of Chemistry and Chemical Engineering, Babeş-Bolyai University, Cluj-Napoca, Romania Department of Chemistry, University of Georgia, Cedar Street, Athens, Georgia 30602, United States

‡

S Supporting Information *

ABSTRACT: The complete series of hydrogen-rich six-vertex cyclopentadienyl dimetallaboranes Cp2M2B4H8 (Cp = η5-C5H5; M = Ir, Ru/Os, Re, Mo/W, and Ta), including the experimentally known Ir, Ru, and Re derivatives synthesized by Fehlner and co-workers, have now been examined by density functional theory. The nature of the central M2B4 polyhedra in the lowest energy Cp2M2B4H8 structures relates to the skeletal electron count as determined by the Wade−Mingos rules. Thus, the lowest energy Cp2Ir2B4H8 structures with 16 Wadean skeletal electrons have central pentagonalpyramidal Ir2B4 units similar to that of the known pentagonal-pyramidal B6H10. The lowest energy Cp2M2B4H8 (M = Ru, Os) structures with 14 Wadean skeletal electrons have central capped-tetragonal-pyramidal rather than octahedral M2B4 units. However, isomeric Cp2M2B4H8 (M = Ru, Os) structures with central M2B4 octahedra are found at energies starting at ∼15 kcal/ mol (M = Ru) and ∼10 kcal/mol (M = Os) above the capped-tetragonal-pyramidal global minima. The lowest energy electron poorer Cp2M2B4H8 structures (M = Re, Mo, W, Ta) have central M2B4 bicapped tetrahedra with the metal atoms at the degree 5 vertices. Higher energy Cp2Re2B4H8 structures include capped-tetragonal-pyramidal structures with surface ReRe double bonds and a pentagonal-pyramidal structure with a surface ReRe triple bond. The lowest energy Cp2M2B4H8 (M = Mo, W) structures appear to have surface MM double bonds and thus also the 12 skeletal electrons for their bicapped-tetrahedral structures. However, the lowest energy likewise bicapped-tetrahedral Cp2Ta2B4H8 structure is best interpreted in having CpTa units with 16-electron rather than 18-electron tantalum configurations and a surface Ta−Ta single bond.

1. INTRODUCTION Hawthorne and co-workers1 first showed that the vertices in the closo deltahedral boranes and related carboranes2,3 could be replaced by isolobal transition-metal vertices, typically units of the type CpM and M(CO)3 (Cp = η5-cyclopentadienyl; M = transition metal). The original metallaboranes and metallacarboranes were found to have the same “most spherical” deltahedral structures as the BnHn2− (n = 6−12) dianions. With the exception of the B11H112− deltahedron, the most spherical deltahedra are characterized by the presence of only degree 4 and degree 5 vertices, where the degree of a vertex is characterized by the number of edges meeting at the vertex in question. Application of the Wade−Mingos rules4−6 for skeletal electron counting to such deltahedral systems was found to lead to to 2n + 2 Wadean skeletal electrons, similar to the corresponding BnHn2− derivatives for the most stable such structures. The subsequent development of metallaborane chemistry, particularly research by Kennedy and co-workers involving the second- and third-row transition metals,7−10 was the discovery of deltahedral metallaborane structures based on deltahedra topologically distinct from the closo deltahedra, called either isocloso11 or hypercloso12−14 deltahedra. Such isocloso metallaborane deltahedra are derived from the closo metal-free borane deltahedra by one or more diamond−square−diamond processes leading to less spherical structures. These structures © XXXX American Chemical Society

typically have a degree 6 vertex for a transition-metal unit. The isocloso deltahedra are electron poor relative to the closo deltahedra, since they have only 2n Wadean skeletal electrons. Further research in metallaborane chemistry, particularly from the laboratory of Fehlner and co-workers,15,16 led to the discovery of still electron poorer dimetallaboranes having only 2n − 4 Wadean skeletal electrons. This family of deltahedra is characterized by highly oblate (flattened) discuslike structures with the two metal atoms at degree 6 or even degree 7 vertices in the flattened direction at the sites of the lowest surface curvature. Such deltahedra are conveniently called oblatocloso deltahedra, to differentiate them from the closo and isocloso deltahedra. The most complete set of stable known compounds exhibiting oblatocloso deltahedral structures are found in the cyclopentadienylrhenium dimetallaboranes Cp2Re2Bn−2Hn−2 (8 ≤ n ≤ 12; Cp = an η5-cyclopentadienyl ligand, most commonly η5Me5C5).17−19 Metal-free polyhedral borane chemistry includes not only the closed deltahedral borane structures BnHn2− and the isoelectronic carboranes CBn−1Hn− and C2Bn−2Hn but also the hydrogen-rich series of neutral boranes BnHn+4 (n = 5, 6, 8, 10) and BnHn+6 (n = 4−9), called nido and arachno boranes, Received: August 4, 2014

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dx.doi.org/10.1021/om500801e | Organometallics XXXX, XXX, XXX−XXX

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Figure 1. Removal of a vertex in various ways from seven-vertex deltahedra analogous to closo, isocloso, and oblatocloso deltahedra.

respectively. In fact, the discovery of some of these neutral hydrogen-rich boranes predates the discovery of the deltahedral borane dianions BnHn2−, since the former are simple neutral binary compounds of boron and hydrogen. The boron frameworks of the n-vertex nido boranes can be derived from the most spherical (n + 1)-vertex closo borane deltahedra by removal of a BH vertex, leaving a “hole” in the polyhedral surface corresponding to an open face with four or more edges. Such nvertex nido polyhedra retain the 2(n + 1) + 2 Wadean skeletal electrons of the (n + 1)-vertex deltahedra from which they are derived and thus are 2n + 4 skeletal electron systems. The “extra” hydrogen atoms in the n-vertex binary nido boranes BnHn+4 are located in the open face generated by removal of the boron vertex from the original (n + 1)-vertex closo deltahedron. The process of generating an n-vertex open nido-like polyhedron by removal of a boron vertex from an (n + 1)-vertex deltahedron can be applied not only to the most spherical closo deltahedra but also to (n + 1)-vertex isocloso and oblatocloso deltahedra. Figure 1 illustrates such processes of vertex removal from seven-vertex deltahedra to give various versions of open sixvertex polyhedra. Thus, the pentagonal bipyramid is the most spherical seven-vertex deltahedron. Removal of a degree 5 vertex from the pentagonal bipyramid gives the pentagonal pyramid. In borane chemistry the known hexaborane B6H10 has such pentagonal-pyramidal geometry.20 A seven-vertex deltahedron is too small to be a true isocloso deltahedron with a higher degree vertex for a transition-metal atom. However, the capped octahedron, such as found in the osmium cluster21 Os7(CO)21, has 14 Wadean skeletal electrons (=2n for n = 7), similar to the true isocloso deltahedra with 9−11 vertices. Removal of a degree 4 vertex from the capped octahedron gives a tetragonal prism capped on a triangular face. The closest seven-vertex deltahedron to an oblatocloso deltahedron is a squashed pentagonal bipyramid with the antipodal degree 5 vertices for the metal atoms. Removal

of a degree 4 vertex from such a deltahedron generates a bicapped tetrahedron if the original pentagonal bipyramid is squashed enough to have the axial metal atoms within bonding distance. This polyhedron is found experimentally in some Cp2M2B4H8 derivatives. The research discussed in this paper uses density functional theory to explore the effect of skeletal electron count on the preferred structures of the hydrogen-rich Cp2M2B4H8 derivatives (Cp = η5-C5H5) using all of the third-row transition metals from tantalum to iridium. The second-row transition metals molybdenum and ruthenium are also included in this study, since related experimental work is available on these metals. Thus, the analogous pentamethylcyclopentadienyl derivatives Cp*2M2B4H8 (Cp* = η5-Me5C5; M = Re,22 Ru,23 Ir24,25) were first synthesized by Fehlner and co-workers and structurally characterized by X-ray crystallography. The molybdenum compound Cp*2Mo2B4H8 has been shown to be an intermediate in reactions of Cp*MoCl4 with LiBH4, followed by treatment with the chalcogen derivatives REER (E = S, Se).26 Subsequent to their syntheses, the chemistry of Cp2M2B4H8 derivatives and related hydrogen-rich M2B4 clusters has received considerable attention.27−38 In addition, related hydrogen-rich Cp2M2B5H9 clusters with a central seven-vertex M2B5 unit have been studied extensively.39−46 The range of second- and third-row transition metals chosen for this study provides systems with skeletal electron counts corresponding to different open nido-like polyhedra derived from the three types of deltahedra depicted in Figure 1. Thus, the Cp2Ir2B4H8 system has 16 Wadean skeletal electrons (=2n + 2 for n = 7), like the pentagonal bipyramid from which it is derived. In fact, Cp2Ir2B4H8 is a transition-metal analogue of the known hexaborane20 B6H10 and is found experimentally to have a similar pentagonal-pyramidal structure.24 In addition, the Cp2M2B4H8 (M = Ru, Os) systems have the 14 Wadean skeletal electrons B


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metal−metal edges, metal−boron edges, and boron−boron edges, respectively.

(=2n for n = 7) for the capped-octahedral structures from which they are derived. The ruthenium derivative Cp2Ru2B4H8 is found experimentally to have the capped-tetragonal-pyramidal structure obtained by removal of a degree 4 vertex from a capped octahedron.23 The Cp2Re2B4H8 system has the 12 Wadean skeletal electrons (=2n − 2 for n = 7) consistent with their origins by removal of a degree 4 vertex from a flattened pentagonal bipyramid, now considered as an oblatocloso structure. The resulting Cp2Re2B4H8 polyhedron is the bicapped tetrahedron in Figure 1. A major difficulty in this work is the large number of possible ways of arranging the four “extra” hydrogen atoms in the Cp2M2B4H8 structures around the underlying M2B4 framework. In order to make these systems tractable, we limited this study to starting structures in which the extra hydrogen atoms are arranged as bridges around the edges of the “hole” (i.e., nontriangular face) generated by removal of a vertex from the parent seven-vertex deltahedron (Figure 1) or across an M−M polyhedral edge. This corresponds to the features of the known experimental structures, which have the “extra” hydrogen atoms bridging either edges of the open face or metal−metal edges.

3. RESULTS 3.1. Cp2Ir2B4H8 Structures. The Cp2Ir2B4H8 system is isoelectronic with the known hexaborane-10,20 B6H10, since a CpIr vertex is isolobal with a BH vertex and is likewise a donor of two skeletal electrons. Thus, the Cp2Ir2B4H8 system has the 16 Wadean skeletal electrons (=2n + 4 for n = 6) required for a nido six-vertex structure,4−6 which is a pentagonal pyramid similar to the B6H10 structure. All five Cp2Ir2B4H8 structures within 22 kcal/mol of the global minimum Ir2B4-1 have a central Ir2B4 pentagonal-pyramidal framework (Figure 2 and Table 1).

2. THEORETICAL METHODS The initial Cp2M2B4H8 structures were constructed by systematic substitution with metal atoms in three six-vertex polyhedra, namely the bicapped tetrahedron, the capped tetragonal pyramid, and the pentagonal pyramid (Figure 1). The extra four hydrogen atoms are considered as edge-capping atoms on the edges of the tetragonal/ pentagonal open face or on the metal−metal edge. This leads to 45 different starting geometries to be optimized for each metal family, as detailed in Table S1 of the Supporting Information. Full geometry optimizations have been carried out on the Cp2M2B4H8 systems at the B3LYP/6-31G(d)47−50 level for all atoms except the metal, for which the SDD (Stuttgart−Dresden ECP plus DZ) basis set51 has been chosen. The lowest-lying structures were then reoptimized at a higher level: i.e., M06L/6-311G(d,p)/SDD.52 The natures of the stationary points after optimization were checked by calculations of the harmonic vibrational frequencies. If significant imaginary frequencies were found, the optimization was continued by following the normal modes corresponding to imaginary frequencies to ensure that genuine minima were obtained. Normally this resulted in reduction of the molecular symmetry. All calculations were performed using the Gaussian 09 package53 with the default settings for the SCF cycles and geometry optimization, namely the fine grid (75302) for numerically evaluating the integrals, 10−8 hartree for the self-consistent field convergence, maximum force of 0.000450 hartree/bohr, RMS force of 0.000300 hartree/bohr, maximum displacement of 0.001800 bohr, and RMS displacement of 0.001200 bohr. Wiberg bond indices (WBIs) for the M−M interactions in the optimized Cp2M2B4H8 structures determined using NBO analysis54 were used, since they are well-established as means for evaluating M−M interactions in polyhedral dimetallaboranes55 as well as other binuclear and trinuclear transition-metal complexes.56 The structures, total and relative energies (including zero-point corrections), and relevant interatomic distances for all calculated systems are given in the Supporting Information. Structures are numbered as M2B4-x where x is the relative order of the structure on the energy scale. Only the lowest energy and thus potentially chemically significant structures (Figures 2−6 and Tables 1−5) are considered in detail in this paper. However, more comprehensive lists of structures, including higher energy structures, are given in the Supporting Information. Most of the Cp2M2B4H8 structures reported in this paper have four terminal hydrogen atoms (one on each boron atom) and four bridging hydrogen atoms. In the tables the locations of the bridging hydrogen atoms are designated as M2, MB, and B2 for hydrogen atoms bridging

Figure 2. Five Cp2Ir2B4H8 structures within 22 kcal/mol of the global minimum.

The four lowest energy Cp2Ir2B4H8 structures (Ir2B4-1, Ir2B4-2, Ir2B4-3, and Ir2B4-4) have adjacent iridium atoms with one iridium atom occupying the apex of the pyramid and the other iridium atom occupying the base of the pyramid. The Ir−Ir distances in all of these structures are ∼2.7 Å with Wiberg bond indices (WBIs) of ∼0.4. These Ir−Ir distances and WBIs are similar to those previously found55 in the closo diiridadicarbaborane structures Cp2Ir2C2BnHn+2. The four Cp2Ir2B4H8 structures differ only in the locations of the “extra” four hydrogen atoms. The lowest energy structure Ir2B4-1 is the only one of these four structures with only three bridging hydrogen atoms, all of which bridge B−B edges of the pentagonal pyramid. The fourth “extra” hydrogen atom is a terminal hydrogen atom bonded to the basal iridium atom. The next three structures Ir2B4-2, Ir2B4-3, and Ir2B4-4 have four bridging hydrogen atoms. In Ir2B4-2 and Ir2B4-3, lying ∼13 kcal/mol in energy above Ir2B4-1, two of the “extra” hydrogen atoms bridge Ir−B edges and the other two bridge B−B edges. However, in Ir2B4-2, lying 15.1 kcal/mol in energy above Ir2B4-1, all four “extra” hydrogen atoms bridge B− B edges. The fifth Cp2Ir2B4H8 structure Ir2B4-5, lying 15.2 kcal/mol in energy above Ir2B4-1, also has a central Ir2B4 framework but with the iridium atoms in nonadjacent positions in the pentagonal base of the pyramid. This leads to a long Ir···Ir distance of ∼3.7 Å and a correspondingly low WBI of 0.09. The permethylated derivative Cp*2Ir2B4H8 (Cp* = η5-Me5C5) has been synthesized and shown by NMR and X-ray crystallography to have the lowest energy structure Ir2B4-1 C


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Table 1. Optimized Cp2Ir2B4H8 Structures within 22 kcal/mol of the Global Minimum with Relative Energies in kcal/mol Ir−Ir structure (symmetry)

rel energy

H bridge

location

distance (Å)

WBI

polyhedron

Ir2B4-1 (Cs) Ir2B4-2 (Cs) Ir2B4-3 (Cs) Ir2B4-4 (Cs) Ir2B4-5 (C1)

0.0 12.8 13.2 15.1 15.2

3B2 2IrB/2B2 2IrB/2B2 4B2 2IrB/2B2

apex−base apex−base apex--base apex−base base−base

2.712 2.715 2.696 2.736 3.742

0.35 0.40 0.39 0.35 0.09

pentagonal pyramid pentagonal pyramid pentagonal pyramid pentagonal pyramid pentagonal pyramid

basal edges of the tetragonal pyramid: namely, two M−B edges and one B−B edge. The M−M distances of 2.828 Å (M = Ru) and 2.889 Å (M = Os) with corresponding WBIs of ∼0.3 in M2B4-1 are similar to those found in the closo Cp2M2C2BnHn+2 (M = Rh, Ir) structures.55 The analogous pair of structures Ru2B4-3 and Os2B4-4, lying ∼12 kcal/mol in energy above M2B4-1, are very similar to M2B4-1 except for a different arrangement of the hydrogen atoms bridging M−B bonds. The next Cp2M2B4H8 (M = Ru, Os) structures in terms of energy M2B4-2, lying ∼8 kcal/mol above M2B4-1, also have a central M2B4 capped tetragonal pyramid but with a rather different arrangement of the metal atoms and the “extra” hydrogen atoms (Figure 3 and Table 2). Thus, the two metal atoms in M2B4-2 form an unbridged M−M edge linking the apical vertex of the underlying tetragonal pyramid with a basal vertex. These M−M distances in M2B4-2 of 2.738 Å (M = Ru) and 2.799 Å (M = Os) are ∼0.1 Å shorter than the M−M distances in the corresponding M2B4-1 derivatives with correspondingly higher WBIs of ∼0.4. The four “extra” hydrogen atoms in M2B4-2 bridge the four edges of the tetragonal face of the capped tetragonal pyramid, leading to two bridged M−B edges and two bridged B−B edges. The central M2B4 capped tetragonal pyramid in the analogous structure pairs Ru2B4-4/ Os2B4-6 and M2B4-5 (M = Ru, Os), lying ∼13 kcal/mol in energy above M2B4-1, is analogous to that in M2B4-2 with the M−M edge connecting the apical vertex to a basal vertex of the tetragonal bipyramid. The three structure sets M2B4-2, Ru2B44/Os2B4-6, and M2B4-5 (M = Ru, Os) differ only in the arrangement of the four “extra” hydrogen atoms. Two higher energy structure types for Cp2M2B4H8 (M = Ru, Os) were found to have the central M2B4 octahedron suggested by the Wade−Mingos rules4−6 for these 14-skeletal-electron systems. Structure Ru2B4-6, lying 15.1 kcal/mol in energy above Ru2B4-1, has three Ru−B edges bridged by hydrogen atoms. The fourth “extra” hydrogen atom forms an unsymmetrical bridge across the Ru−Ru edge with short and long Ru−H distances of 1.64 and 2.04 Å, respectively. In the analogous Cp2Os2B4H8 structure Os2B4-3, lying 10.3 kcal/mol in energy above Os2B4-1, the hydrogen bridge across the Os−Os edge is even more unsymmetrical, with short and long Os−H distances of 1.63 and 2.48 Å, respectively. This long Os−H distance is so long that it is effectively nonbonding, thereby making this hydrogen atom effectively a terminal hydrogen atom. The permethylated derivative Cp*2Ru2B4H8 has been synthesized and shown by X-ray crystallography to have a structure analogous to the lowest energy Cp2Ru2B4H8 structure Ru2B4-1 (Figure 3 and Table 2).23 The experimental Ru−Ru distance of 2.8550(7) Å is close to the predicted value of 2.828 Å for Ru2B4-1. Similarly the experimental Ru−B and B−B distances in the open tetragonal Ru2B2 face of Cp*2Ru2B4H8 of ∼2.22 and 1.834 Å, respectively, are close to the predicted values of 2.244 and 1.796 Å, respectively, for Ru2B4-1.

with the expected pentagonal-pyramidal Ir2B4 framework having adjacent iridium atoms.24 The experimental Ir−Ir distance of 2.738(7) Å is close to the 2.712 Å Ir−Ir distance predicted for Ir2B4-1 (Table 1). 3.2. Cp2M2B4H8 (M = Ru, Os) Structures. The Cp2M2B4H8 (M = Ru, Os) systems have 14 Wadean skeletal electrons, since each CpM vertex is a donor of only one skeletal electron. This could correspond to a closo octahedral M2B4 framework. However, the central M2B4 units in the five lowest energy Cp2Ru2B4H8 structures and their Cp2Os2B4H8 analogues are not octahedral but instead M2B3 tetragonal pyramids, with one of the triangular faces capped by the fourth boron atom (Figure 3 and Table 2). The open tetragonal faces of such structures provide suitable locations for the “extra” hydrogen atoms as edgebridging hydrogens. The lowest energy Cp2M2B4H8 (M = Ru, Os) structures M2B4-1 have a hydrogen-bridged M−M bond in the tetragonal base of the underlying tetragonal pyramid (Figure 3 and Table 2). The remaining three “extra” hydrogen atoms bridge the other

Figure 3. Seven Cp2M2B4H8 (M = Ru, Os) structures within 17 kcal/ mol of the global minimum. D


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Table 2. Optimized Cp2M2B4H8 (M = Ru, Os) Structures within 17 kcal/mol of the Global Minimum with Relative Energies in kcal/mol Cp2Ru2B4H8

Cp2Os2B4H8

structure (symmetry)

polyhedron

H bridge

ΔE

Ru−Ru distance (Å)

WBI

ΔE

Os−Os distance (Å)

WBI

Ru2B4-1/Os2B4-1 (Cs) Ru2B4-2/Os2B4-2 (C1) Ru2B4-3/Os2B4-4 (C1) Ru2B4-4/Os2B4-6 (C1) Ru2B4-5/Os2B4-5 (C1) Ru2B4-6/Os2B4-3 (C1) Ru2B4-7/Os2B4-7 (C2)

cap tetrag pyr cap tetrag pyr cap tetrag pyr cap tetrag pyr cap tetrag pyr octahedron octahedron

M2/2MB/B2 2MB/B2 M2/2MB/B2 M2/MB/2B2 M2/3MB M2/3MB M2/3MB

0.0 8.6 12.6 13.2 13.4 15.1 16.2

2.828 2.738 2.874 2.732 2.733 2.762 2.838

0.30 0.39 0.29 0.33 0.34 0.36 0.29

0.0 7.6 12.2 14.4 13.4 10.3 14.7

2.889 2.799 2.939 2.783 2.790 2.794 2.902

0.32 0.40 0.30 0.36 0.36 0.46 0.31

system. The Re−Re distances of ∼2.8 Å and corresponding WBIs of ∼0.5 in Re2B4-1 and Re2B4-2 are similar to those in the oblatocloso derivatives Cp2Re2BnHn.58 This can be related to the relationship of Re2B4-1 and Re2B4-2 to the experimentally known 19 oblatocloso hexagonal-bipyramidal structure Cp2Re2B6H6 by removing two adjacent BH groups and bridging the four Re−B edges of the resulting “hole” with the four “extra” hydrogen atoms. The next higher energy Cp2Re2B4H8 structure, namely Re2B4-3 lying 10.2 kcal/mol in energy above Re2B4-1, also has a bicapped tetrahedral geometry of the central Re2B4 unit with a similar ReRe distance of 2.846 Å and WBI of 0.48. In Re2B4-3 two of the “extra” hydrogens bridge symmetryequivalent Re−B edges and a third “extra” hydrogen atom bridges a B−B edge. The remaining “extra” hydrogen is a terminal hydrogen bonded to a rhenium atom. The permethylated version of Re2B4-1, namely Cp*2Re2B4H8, has been synthesized and characterized structurally by X-ray crystallography.22 The experimental Re−Re distance of 2.8091 Å is close to the theoretical value of 2.832 Å for Re2B4-1. Similarly, the experimental B−B distances of 1.74, 1.94, and 1.64 Å in the B4 chain can be compared with the theoretical values of 1.820, 1.637, and 1.693 Å for Re2B4-1. The discrepancies between the experimental B−B distances in Cp*2Re2B4H8 and the predicted B−B distances in Re2B4-1 can be related to the effects of the methyl substituents in Cp*2Re2B4H8 and difficulties in the experimental determination of accurate B−B distances by X-ray crystallography in the presence of two heavy rhenium atoms. The next three Cp2Re2B4H8 structures in terms of energy provide examples of surface rhenium−rhenium multiple bonding. Structures Re2B4-4 and Re2B4-6, lying 16.1 and 22.1 kcal/mol, respectively, above Re2B4-1, have a central Re2B4 capped tetragonal pyramid with adjacent rhenium atoms sharing an edge bridged by one of the “extra” hydrogen atoms (Figure 4 and Table 3). In Re2B4-4 the ReRe edge of length 2.596 Å with a corresponding WBI of 0.78 connects the apical vertex of the pyramid with a basal vertex. However, in Re2B4-6 the ReRe edge of length 2.721 Å with a corresponding WBI of

3.3. Cp2Re2B4H8 Structures. The Cp2Re2B4H8 system has 12 Wadean skeletal electrons, since each CpRe vertex is a formal donor of 0 skeletal electrons. This corresponds to a bicappedtetrahedral geometry such as that found experimentally in Os6(CO)18, also a 12-skeletal-electron system.57 The lowest energy Cp2Re2B4H8 structure Re2B4-1 has Cs symmetry with bicapped-tetrahedral geometry of the central Re2B4 unit (Figure 4 and Table 3). All four “extra” hydrogen atoms in Re2B4-1

Figure 4. Six Cp2Re2B4H8 structures within 25 kcal/mol of the global minimum.

bridge Re−B edges to degree 3 boron vertices at the ends of a B4 chain. In the more symmetrical C2v Cp2Re2B4H8 structure Re2B4-2 the B4 chain connecting the two rhenium atoms has broken into two separate B2 units while retaining the four hydrogen-bridged Re−B edges. Structures Re2B4-1 and Re2B42 lie only 2.6 kcal/mol apart in energy, suggesting a fluxional

Table 3. Optimized Cp2Re2B4H8 Structures within 25 kcal/mol of the Global Minimum with Relative Energies in kcal/mol Re−Re structure (symmetry)

rel energy

H bridge

location

distance (Å)

WBI

polyhedron

Re2B4-1 (Cs) Re2B4-2 (C2v) Re2B4-3 (Cs) Re2B4-4 (C1) Re2B4-5 (Cs) Re2B4-6 (Cs)

0.0 2.6 10.2 16.1 20.4 22.1

4ReB 4ReB 2ReB/B2 Re2/2ReB/B2 Re2/3B2 Re2/2ReB/B2

2 apical 2 apical 2 apical apex−base apex−base base−base

2.832 2.806 2.846 2.596 2.458 2.721

0.49 0.52 0.48 0.78 1.18 0.84

bicap tetrahedron separate B2 ligands bicap tetrahedron cap tetrag pyr pentagonal pyramid cap tetrag pyr

E


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2/W2B4-1 one of the “extra” hydrogen atoms bridges the M M double bond. The presence of the hydrogen bridge shortens the MM distance from ∼3.0 Å in Mo2B4-1/W2B4-2 to ∼2.6 Å in Mo2B4-2/W2B4-1. The next two low-energy Cp2M2B4H8 (M = Mo, W) structures M2B4-3 and M2B4-4 have central M2B3 tetragonal pyramids with one of the triangular faces capped by a BH group, similar to the geometry of the lowest energy Cp2M2B4H8 (M = Ru, Os) deerivatives (Figure 5 and Table 4). In the structures M2B4-3 (M = Mo, W), lying ∼9 kcal/mol above M2B4-1, the MM edges are located at the base of the tetragonal pyramid. However, in the higher energy structures M2B4-3 (M = Mo, W), lying ∼22 kcal/mol above M2B4-1, the MM edges connect the base with the apex of the tetragonal pyramid. In both structure types M2B4-3 and M2B4-4 (M = Mo, W) the short MM distances of ∼2.5 Å with correspondingly large WBIs of ∼1.4 suggest formal triple surface bonds, which donate an “extra” four skeletal electrons. Thus, the M2B4-3 and M2B4-4 (M = Mo, W) structures become 14 Wadean skeletal electron systems ,consistent with their tetragonal pyramidal geometry and similar to the lowest energy Cp2M2B4H8 (M = Ru, Os) structures (Figure 3 and Table 2) having the same capped-tetragonalpyramidal geometries. 3.5. Cp2Ta2B4H8 Structures. The Cp2Ta2B4H8 energy surface appears to be significantly simpler than the other Cp2M2B4H8 energy surfaces, since only three structures were found within 22 kcal/mol of the global minimum Ta2B4-1 (Figure 6 and Table 5). Furthermore, Ta2B4-1 lies

0.84 connects two adjacent vertices of the tetragonal basal face of the pyramid. These ReRe edge lengths and WBI values suggest formal double surface bonds. Such surface ReRe double bonds contribute an extra two skeletal electrons so that Re2B4-4 and Re2B4-6 each have 14 Wadean skeletal electrons similar to the capped-tetragonal-pyramidal Cp2Os2B4H8 structures discussed above. The other low-energy Cp2Re2B4H8 structure Re2B4-5, lying 20.4 kcal/mol above Re2B4-1, has a central Re2B4 pentagonal pyramid (Figure 4 and Table 3). In Re2B4-5 the ReRe edge of length 2.458 Å with a corresponding WBI of 1.18 connects the apical vertex of the pyramid with a basal vertex. This short Re Re distance and corresponding high WBI suggests a formal triple surface bond. Such surface ReRe triple bonds contribute an extra four skeletal electrons so that Re2B4-5 has 16 Wadean skeletal electrons similar to the pentagonal-pyramidal Cp2Ir2B4H8 nido structures discussed above. 3.4. Cp2M2B4H8 (M = Mo, W) Structures. The two lowest energy Cp2M2B4H8 (M = Mo, W) structures M2B4-1 and M2B4-2 both have bicapped-tetrahedral geometry with the metal atoms at the degree 5 vertices (Figure 5 and Table 4).

Figure 5. Four Cp2M2B4H8 (M = Mo, W) structures within 25 kcal/mol of the global minimum.

Their energies are very close (within ∼1 kcal/mol), suggesting a fluxional system involving movement of the “extra” hydrogen atoms. The WBIs of ∼0.9 for the M−M edges of the bicapped tetrahedra in M2B4-1 and M2B4-2 (M = Mo, W) are nearly double the ∼0.5 WBIs for the Re−Re edges in the bicappedtetrahedral Cp2Re2B4H8 structures Re2B4-1, Re2B4-2, and Re2B4-3 (Table 3), suggesting formal MM surface double bonds in M2B4-1 and M2B4-2 (M = Mo, W). The extra two skeletal electrons from the MM surface double bonds balance the two single electron acceptor CpM (M = Mo, W) vertices (i. e., −1 skeletal electron donors) so that these Cp2M2B4H8 (M = Mo, W) structures have the 12 Wadean skeletal electrons for a bicapped tetrahedron. In the structure pair Mo2B4-1/W2B4-2 all four “extra” hydrogen atoms bridge the M−B edges similarly to Re2B4-1, thus connecting the degree 5 metal vertices to the degree 3 boron vertices. However, in the structure pair Mo2B4-

Figure 6. Two Cp2Ta2B4H8 structures within 22 kcal/mol of the global minimum.

14.9 kcal/mol in energy below the next lowest energy structure Ta2B4-2, so that it appears to be highly favored. All three structures have bicapped-tetrahedral geometries. However, the two higher energy structures Ta2B4-2 and Ta2B4-3 have the unusual feature of a tantalum atom at a degree 4 vertex rather than both tantalum atoms at degree 5 vertices, similar to the case of all of the other bicapped-tetrahedral Cp2M2B4H8 structures discussed in this paper.

Table 4. Optimized Cp2M2B4H8 (M = Mo, W) Structures within 25 kcal/mol of the Global Minimum with Relative Energies in kcal/mol Cp2Mo2B4H8

Cp2W2B4H8

structure (symmetry)

polyhedron

H bridge

ΔE

Mo−Mo distance (Å)

WBI

ΔE

W−W distance (Å)

WBI

Mo2B4-1/W2B4-2 (C2v) Mo2B4-2/W2B4-1 (C1) Mo2B4-3/W2B4-3 (Cs) Mo2B4-4/W2B4-4 (C1)

bicap tetrahedron bicap tetrahedron cap tetrag pyr cap tetrag pyr

4MB M2/3MB M2/2MB/B2 M2/2MB/B2

0.0 1.1 8.4 21.3

2.997 2.604 2.495 2.476

0.87 0.95 1.51 1.39

0.7 0.0 9.8 23.1

3.035 2.632 2.528 2.518

0.88 0.97 1.52 1.34

F


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Table 5. Optimized Cp2Ta2B4H8 Structures within 22 kcal/mol of the Global Minimum with Relative Energies in kcal/mol Ta−Ta structure (symmetry)

rel energy

H bridge

location

distance (Å)

WBI

polyhedron

Ta2B4-1 (C2v) Ta2B4-2 (C1) Ta2B4-3 (Cs)

0.0 14.9 19.4

4TaB Ta2/3TaB 2Ta2/2TaB

2 deg 5 deg 5/deg 4 deg 5/deg 4

2.977 2.724 2.686

0.76 1.10 1.07

bicap tetrahedron bicap tetrahedron bicap tetrahedron

BH vertices, so that Cp2Ir2B4H8 has the nido 2n + 4 skeletal electron count of 16 (n = 6) and thus is the direct analogue of hexaborane-10, known to have a pentagonal-pyramidal structure.20 Furthermore, all five of the lowest energy Cp2Ir2B4H8 structures have a pentagonal-pyramidal geometry. In addition, structures with direct Ir−Ir interactions are preferred, since the lowest energy structure Ir2B4-5 with nonadjacent iridium atoms lies ∼15 kcal/mol above the global minimum Ir2B4-1. Now consider the ruthenium and osmium derivatives Cp2M2B4H8 (M = Ru, Os) (Figure 3 and Table 2). The CpM (M = Ru, Os) vertices contribute 8 − 7 = 1 skeletal electron so that these systems have the 2n + 2 skeletal electron count of 14 (n = 6) for the 6-vertex closo deltahedron: namely, the regular octahedron. However, the regular octahedron does not have an open (i.e., nontriangular) face for the “extra” hydrogen atoms. Therefore, five of the six lowest energy Cp2M2B4H8 (M = Ru, Os) structures do not have octahedral geometries but instead tetragonal-pyramidal geometries capped on a triangular face (Figure 1). The tetragonal base of the central pyramid in such Cp2M2B4H8 geometries provides an open face for distribution of the four “extra” hydrogen atoms. The central tetragonal pyramid in these Cp2M2B4H8 (M = Ru, Os) structures is a nido structure, derived from the regular octahedron by removal of a vertex, and thus also requires 14 skeletal electrons. Two of these skeletal electrons are provided by the BH group capping one of the four triangular faces of the tetragonal pyramid. The face capped by this extra BH group contains both metal atoms in order to have enough external orbitals for strong bonding to the BH cap. Octahedral Cp2M2B4H8 (M = Ru, Os) structures are also found, but at higher energies than the capped-tetragonal-pyramidal structures. Known dirhenaborane structures include a homologous series of oblatocloso derivatives Cp2Re2BnHn (n = 6−10).16−19 Such structures have the rhenium atoms at antipodal high degree vertices of a deltahedron, which is flattened enough to have these antipodal rhenium atoms within bonding distance. The smallest such oblatocloso dirhenaborane that has been realized experimentally is the hexagonal-bipyramidal Cp*2Re2B6H6, isolated as a halogenated derivative.19 A flattened pentagonal bipyramid, which has not been experimentally realized in Cp2Re2B5H5, would appear to the next lower member of this series of oblatocloso dirhenaboranes. Removal of an equatorial vertex from a flattened pentagonal bipyramid or two adjacent equatorial vertices from the experimentally realized hexagonal bipyramid gives a polyhedral structure found experimentally in Cp*2Re2B4H8 and predicted to be the lowest energy structures for Cp2M2B4H8 (M = Re, Mo, W, Ta). Since the antipodal metal atoms in the original flattened-hexagonal or pentagonal bipyramid are within bonding distance, this internal bond between antipodal vertices becomes an external polyhedral edge upon removal of the equatorial vertex/vertices. The resulting polyhedron is a bicapped tetrahedron similar to that found in the known Os6(CO)18.57 Since a CpRe vertex contributes 7 − 7 = 0 skeletal electrons, the Cp2Re2B4H8 structures have 12 Wadean

The bicapped-tetrahedral geometry of the lowest energy Cp2Ta2B4H8 structure Ta2B4-1 is similar to that of the lowest energy Cp2M2B4H8 (M = Re, Mo, W) structures with the metal atoms at the degree 5 vertices (Figure 6 and Table 5). Extrapolating from the surface Re−Re single bond in the Cp2Re2B4H8 structures Re2B4-1, Re2B4-2, and Re2B4-3 and the surface MM double bond in the Cp2M2B4H8 (M = Mo, W) structures M2B4-1 and M2B4-2 would suggest a surface TaTa triple bond in the Cp2Ta2B4H8 structure Ta2B4-1 to provide the 12 skeletal electrons for a bicapped tetrahedron with the favored 18-electron configuration for the tantalum atoms. However, the Ta−Ta distance of 2.977 Å with a WBI of 0.76 is closer to that expected for a formal single bond. A CpTa vertex with a 16electron configuration is a donor of 0 skeletal electrons, similar to the case of a CpRe vertex with the favored 18-electron configuration. Therefore, a Cp2Ta2B4H8 structure having tantalum atoms with 16-electron rather than 18-electron configurations can have the 12 Wadean skeletal electrons for a bicapped tetrahedron similar to the Cp2Re2B4H8 structures having rhenium atoms with the favored 18-electron configuration. Early-transition-metal derivatives with 16-electron rather than 18-electron configurations are frequently stable species, as exemplified by the titanium derivatives Cp2TiX2 (X = halide, alkyl, aryl, etc.),59−61 (η5-C5H5)(η7-C7H7)Ti,62 and (η6CH3C6H5)2Ti.63

4. DISCUSSION The effect of skeletal electron count on the preferred M2B4 polyhedra in the lowest energy Cp2M2B4H8 structures can be rationalized by the Wade−Mingos rules.4−6 Application of these rules assumes somewhat artificially that each CpM vertex provides three orbitals for skeletal bonding. Assuming a 9-orbital sp3d5 manifold makes a CpM vertex a donor of g − 7 skeletal electrons, where g is the group number of M in the periodic table. This “g − 7 rule” derives from the need for 6 electrons for lone pairs in 3 of the 9 sp3d5 manifold orbitals of the metal M not used for external bonds plus an additional electron removed in reducing the C5H5 ring to the cyclopentadienide anion. Note that 3 of the 9 orbitals of the metal sp3d5 manifold are required for the η5 bonding of the Cp ring to the metal atom. The three polyhedra found in the lowest energy Cp2M2B4H8 structures are the pentagonal bipyramid, the capped square pyramid, and the bicapped tetrahedron, in order of decreasing Wadean skeletal electron requirement (Figure 1). These three polyhedra are all found in experimental Cp*2M2B4H8 structures containing the pentamethylcyclopentadienyl ligand. Thus, Cp*2Ir2B4H8, Cp*2Ru2B4H8, and Cp*2Re2B4H8 are the experimentally realized representatives of pentagonal-bipyramidal,22 capped-tetragonal-pyramidal,23 and bicapped-tetrahedral24 structures, respectively. In all cases the theoretically predicted lowest energy structures correspond to these experimental structures within experimental and computational errors. Consider first the pentagonal-pyramidal lowest energy Cp2Ir2B4H8 structure Ir2B4-1 (Figure 2 and Table 1). The CpIr vertices contribute 9 − 7 = 2 skeletal electrons, similar to G


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skeletal bonding in Ta2B4-1 has a 16-electron configuration for the tantalum atoms in the CpTa vertices and a normal surface Ta−Ta single bond. Such 16-electron configurations are not unusual for early-transition-metal derivatives requiring large numbers of external ligands to achieve the favored 18-electron configuration. This interpretation gives Ta2B4-1 the 12 Wadean skeletal electrons for its bicapped-tetrahedral structure.

skeletal electrons, similar to Os6(CO)18. These structures are characterized by Re−Re distances of ∼2.8 Å and WBIs of ∼0.5. Another structural feature for dirhenaboranes, predicted theoretically but not yet experimentally realized, is the presence of surface multiple Re−Re bonds. Such surface multiple Re−Re bonds contribute an extra 2(b −1) skeletal electrons, where b is the formal bond order. Surface multiple Re−Re bonds were first predicted to occur in higher energy deltahedral dirhenaborane Cp2Re2BnHn structures.58 They were then predicted to occur in the lowest energy PnRe2BnHn structures, where Pn is the η8-C8H6 pentalene ligand, holding the two rhenium atoms in close proximity as a PnRe2 unit, thereby preventing them from occupying antipodal vertices.64 Some higher energy Cp2Re2B4H8 structures also appear to have surface multiple Re−Re bonds. Thus, the capped-tetragonal-pyramidal Cp2Re2B4H8 structures Re2B4-4 and Re2B4-6 have ReRe distances of 2.6−2.7 Å and WBIs of ∼0.8, suggesting formal double bonds (Figure 4 and Table 3). Considering the extra pair of skeletal electrons from the surface ReRe double bond makes these species 14 Wadean skeletal electron systems, similar to the Wadean skeletal electron count of the capped-tetragonal-pyramidal Cp2M2B4H8 (M = Ru, Os) structures (Figure 3 and Table 2). Furthermore, the pentagonal-pyramidal Cp2Re2B4H8 structure Re2B4-5 has an even shorter ReRe distance of ∼2.46 Å with a corresponding larger WBI of 1.18, suggesting a surface triple bond. Considering the extra two pairs of skeletal electrons from this surface, the ReRe triple bond makes Re2B4-5 a 16 Wadean skeletal electron system, similar to all of the low-energy Cp2Ir2B4H8 structures (Figure 2 and Table 1). The low-energy Cp2M2B4H8 structures of the group 6 metals molybdenum and tungsten also appear all to have surface multiple metal−metal bonds (Figure 5 and Table 4). The two lowest energy Cp2M2B4H8 (M = Mo, W) structures M2B4-1 and M2B4-2 (M = Mo, W) have central M2B4 bicapped tetrahedra with MM WBIs of ∼0.9, suggesting surface double bonds. The CpM (M = Mo, W) vertices are 6 − 7 = −1 electron donors of skeletal electrons: i.e., acceptors of a single skeletal electron. The removal of a skeletal electron by each CpM vertex in these Cp2M2B4H8 (M = Mo, W) systems is counterbalanced by the extra electron pair from the surface M=M double bonds, thereby making M2B4-1 and M2B4-2 12 skeletal electron systems similar to the bicapped-tetrahedral Cp2Re2B4H8 structures with normal surface Re−Re single bonds. The higher energy Cp2M2B4H8 structures B4M2-3 and B4M2-4 (M = Mo, W) have central capped-tetragonal-pyramidal M2B4 units similar to the lowest energy Cp2M2B4H8 (M = Ru, Os) structures (Figure 5 and Table 4). The short MM distances of ∼2.5 Å with corresponding high WBIs of ∼1.4 in these systems correspond to formal surface triple bonds. Thus, B4M2-3 and B4M2-4 (M = Mo, W) are 14-skeletal-electron systems similar to the low-energy Cp2M2B4H8 (M = Ru, Os) structures with similar capped-tetragonal-pyramidal M2B4 units. The tantalum system Cp2Ta2B4H8 is the simplest of the Cp2M2B4H8 systems, since a single structure Ta2B4-1 lies more than 14 kcal/mol below the next lowest energy structure Ta2B42 (Figure 6 and Table 5). The structure Ta2B4-1 has a central Ta2B4 bicapped tetrahedron similar to that for the lowest energy Cp2M2B4H8 (M = Re, Mo, W) structures. A formal surface Ta Ta triple bond is required to provide a bicapped-tetrahedral Ta2B4-1 the 12 Wadean skeletal electrons characteristic of such structures. However, the Ta−Ta distance of ∼3.0 Å appears to be too long and the corresponding WBI of ∼0.8 appears to be too low for a formal triple bond. A more likely interpretation of the

5. SUMMARY The nature of the central M2B4 polyhedra in the lowest energy Cp2M2B4H8 structures relates to the skeletal electron count as determined by the Wade−Mingos rules.4−6 Thus, the experimentally known Cp2Ir2B4H8 system with 16 Wadean skeletal electrons has a pentagonal-pyramidal Ir2B4 unit similar to the known pentagonal-pyramidal B6H10. However, the experimentally known Cp2Ru2B4H8 system and its Cp2Os2B4H8 analogue with 14 Wadean skeletal electrons have capped-tetragonalpyramidal M2B4 units. Isomeric Cp2M2B4H8 (M = Ru, Os) structures with central M2B4 octahedra are found at energies starting at ∼15 kcal/mol (M = Ru) and ∼10 kcal/mol (M = Os) above the capped-tetragonal-pyramidal global minima. The electron-poorer Cp2M2B4H8 systems (M = Re, Mo, W, Ta) have central M2B4 bicapped tetrahedra with the metal atoms at the degree 5 vertices. The experimentally known Cp2Re2B4H8 system has 12 Wadean skeletal electrons, similar to the known bicapped-tetrahedral osmium carbonyl Os6(CO)18. Higher energy Cp2Re2B4H8 structures include capped-tetragonalpyramidal structures with surface ReRe double bonds and a pentagonal-pyramidal structure with a surface ReRe triple bond. The experimentally unknown Cp2M2B4H8 (M = Mo, W) systems appear to have surface MM double bonds and thus also have the 12 skeletal electrons for their bicapped-tetrahedral structures. However, the lowest energy likewise bicappedtetrahedral Cp2Ta2B4H8 structure is best interpreted in having CpTa units with 16-electron rather than 18-electron tantalum configurations and a surface Ta−Ta single bond.

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ASSOCIATED CONTENT

S Supporting Information *

Table S1, giving initial Cp2M2B4H8 structures, Table S2A, giving distances for the lowest-lying Cp2Ir2B4H8 structures, Table S2B, giving the energy ranking for all of the Cp2Ir2B4H8 structures, Table S3A, giving distances for the lowest-lying Cp2Ru2B4H8 structures, Table 3B, giving the energy ranking for all of the Cp2Ru2B4H8 structures, Table S4A, giving distances for the lowest-lying Cp2Os2B4H8 structures, Table S4B, giving the energy ranking for all of the Cp2Os2B4H8 structures, Table S5A, giving the distances for the lowest-lying Cp2Re2B4H8 structures, Table S5B, giving the energy ranking for all of the Cp2Re2B4H8 structures, Table S6A, giving the distances for the lowest-lying Cp2Mo2B4H8 structures, Table S6B, giving the energy ranking for all of the Cp2Mo2B4H8 structures, Table S7A, giving the distances for the lowest-lying Cp2W2B4H8 structures, Table S7B, giving the energy ranking for all of the Cp2W2B4H8 structures, Table S8A, giving the distances for the lowest-lying Cp2Ta2B4H8 structures, Table S8B, giving the energy ranking for all of the Cp2Ta2B4H8 structures, text giving the complete Gaussian09 reference (ref 53), and an xyz file giving structure information. This material is available free of charge via the Internet at http:// pubs.acs.org. H


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(30) Bose, S. K.; Geetharani, K.; Varghese, B.; Mobin, S. M.; Ghosh, S. Chem. Eur. J. 2008, 14, 9058. (31) Bose, S. K.; Geetharani, K.; Ramkumar, V.; Mobin, S. M.; Ghosh, S. Chem. Eur. J. 2009, 15, 13483. (32) Bose, S. K.; Geetharani, K.; Varghese, B.; Ghosh, S. Inorg. Chem. 2010, 49, 6375. (33) Chakrahari, K. K. V.; Mobin, S. M.; Ghosh, S. J. Cluster Sci. 2011, 22, 149. (34) Dhayal, R. S.; Ramkumar, V.; Ghosh, S. Polyhedron 2011, 30, 2062. (35) Bose, S. K.; Mobin, S. M.; Ghosh, S. J. Organomet. Chem. 2011, 696, 3121. (36) Bose, S. K.; Geetharani, K.; Sahoo, S.; Reddy, K. H. R.; Varghese, B.; Jemmis, E. D.; Ghosh, S. Inorg. Chem. 2011, 50, 9414. (37) Thakur, A.; Sao, S.; Ramkumar, V.; Ghosh, S. Inorg. Chem. 2012, 51, 8322. (38) Krishnamoorthy, B. S.; Thakur, A.; Chakrahari, K. K. V.; Bose, S. K.; Hamon, P.; Roisnel, T.; Kahlal, S.; Ghosh, S.; Halet, J.-F. Inorg. Chem. 2012, 51, 10375. (39) Bould, J.; Rath, N. P.; Barton, L. Organometallics 1995, 14, 2119. (40) Aldridge, S.; Hashimoto, H.; Kawamura, K.; Shang, M.; Fehlner, T. P. Inorg. Chem. 1998, 37, 928. (41) Weller, A. S.; Shang, M.; Fehlner, T. P. Organometallics 1999, 18, 53. (42) Ghosh, S.; Beatty, A. M.; Fehlner, T. P. J. Am. Chem. Soc. 2001, 123, 9188. (43) Kim, D. Y.; Girolami, G. S. J. Am. Chem. Soc. 2006, 128, 10969. (44) Sahoo, S.; Reddy, K. H. K.; Dhayal, R. S.; Mobin, S. M.; Ramkumar, V.; Jemmis, E. D.; Ghosh, S. Inorg. Chem. 2009, 48, 6509. (45) Bose, S. K.; Geetharani, K.; Ghosh, S. Chem. Commun. 2011, 47, 11996. (46) Bose, S. K.; Geetharani, K.; Varghese, B.; Ghosh, S. Inorg. Chem. 2011, 50, 2445. (47) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200. (48) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (49) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623. (50) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1998, 37, 785. (51) Andrae, D.; Haussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1990, 77, 123. (52) Truhlar, D. G.; Zhao, Y. Theor. Chem. Acc. 2008, 120, 215. (53) Gaussian 09 (Revision A.02); Gaussian, Inc., Wallingford, CT, 2009. The complete reference is given in the Supporting Information. (54) Weinhold, F.; Landis, C. R. Valency and Bonding: A Natural Bond Order Donor-Acceptor Perspective; Cambridge University Press: Cambridge, U.K., 2005; pp 32−36. (55) Lupan, A.; King, R. B. Inorg. Chim. Acta 2013, 397, 83. (56) Wang, H.; Xie, Y.; King, R. B.; Schaefer, H. F. J. Am. Chem. Soc. 2006, 128, 11376. (57) Mason, R.; Thomas, K. M.; Mingos, D. M. P. J. Am. Chem. Soc. 1973, 95, 3802. (58) Lupan, A.; King, R. B. Inorg. Chem. 2012, 51, 7609. (59) Wilkinson, G.; Birmingham, J. M. J. Am. Chem. Soc. 1954, 76, 4281. (60) Clearfield, A.; Warner, D. K.; Saldarriaga-Molina, C. H.; Ropal, R.; Bernal, I. Can. J. Chem. 1975, 53, 1622. (61) Thewalt, U.; Woehrle, T. J. Organomet. Chem. 1994, 464, C17. (62) Zeinstra, J. D.; De Boer, J. L. J. Organomet. Chem. 1973, 54, 207. (63) Tairova, G. G.; Krasochka, O. N.; Ponomarev, V. I.; Kvashina, E. F.; Shvetsov, Yu. A.; Lisetskii, E. M.; Kiryukhin, D. P.; Atovmyan, L. O.; Borod’ko, Yu. G. Transition Met. Chem. 1982, 7, 189. (64) Lupan, A.; King, R. B. Organometallics 2013, 32, 4002.

AUTHOR INFORMATION

Corresponding Authors

*E-mail for A.L.: [email protected]. *E-mail for R.B.K.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding from the European Social Fund (Grant POSDRU/159/ 1.5/5/137750), the Romanian Ministry of Education and Research (Grant PN-II-ID-PCE-2012-4-0488), and the U.S. National Science Foundation (Grant CHE-1057466) is gratefully acknowledged.

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REFERENCES

(1) Callahan, K. P.; Hawthorne, M. F. Adv. Organomet. Chem. 1976, 14, 145. (2) Williams, R. E. Inorg. Chem. 1971, 10, 210. (3) Williams, R. E. Chem. Rev. 1992, 92, 177. (4) Wade, K. Chem. Commun. 1971, 792. (5) Mingos, D. M. P. Nat. Phys. Sci. 1972, 99, 236. (6) Mingos, D. M. P. Acc. Chem. Res. 1984, 17, 311. (7) Bould, J.; Kennedy, J. D.; Thornton-Pett, M. J. Chem. Soc., Dalton Trans. 1992, 563. (8) Kennedy, J. D.; Štibr, B. In Current Topics in the Chemistry of Boron; Kabalka, G. W., Ed.; Royal Society of Chemistry: Cambridge, U.K., 1994; pp 285−292. (9) Kennedy, J. D. In The Borane-Carborane-Carbocation Continuum; Casanova, J., Ed.; Wiley: New York, 1998; Chapter 3, pp 85−116. (10) Štibr, B.; Kennedy, J. D.; Drdáková, E.; Thornton-Pett, M. J. Chem. Soc., Dalton Trans. 1994, 229. (11) Kennedy, J. D. Inorg. Chem. 1986, 25, 111. (12) Baker, R. T. Inorg. Chem. 1986, 25, 109. (13) Johnston, R. L.; Mingos, D. M. P. Inorg. Chem. 1986, 25, 3321. (14) Johnston, R. L.; Mingos, D. M. P.; Sherwood, P. New J. Chem. 1991, 15, 831. (15) For a review of much of the relevant chemistry from Fehlner’s group see: Fehlner, T. P. In Group 13 Chemistry: From Fundamentals to Applications; Shapiro, P. J., Atwood, D. A., Eds.; American Chemical Society: Washington, DC, 2002; pp 49−67. (16) Fehlner, T. P.; Halet, J.-F. Saillard, J.-Y. Molecular Clusters. A Bridge to Solid State Chemistry; Cambridge University Press: Cambridge, U.K., 2007. (17) Ghosh, S.; Shang, M.; Li, Y.; Fehlner, T. P. Angew. Chem., Int. Ed. 2001, 40, 1125. (18) Wadepohl, H. Angew. Chem., Int. Ed. 2002, 41, 4220. (19) Le Guennic, B.; Jiao, H.; Kahlal, S.; Saillard, J.-Y.; Halet, J.-F.; Ghosh, S.; Shang, M.; Beatty, A. M.; Rheingold, A. L.; Fehlner, T. P. J. Am. Chem. Soc. 2004, 126, 3203. (20) Hirschfeld, F. L.; Eriks, K.; Dickerson, R. E.; Lippert, E. L., Jr.; Lipscomb, W. N. J. Chem. Phys. 1958, 28, 56. (21) Eady, C. R.; Johnson, B. F. G.; Lewis, J.; Mason, R.; Hitchcock, P. B.; Thomas, K. M. Chem. Commun. 1977, 385. (22) Ghosh, S.; Shang, M.; Fehlner, T. P. J. Organomet. Chem. 2000, 614−615, 92. (23) Lei, X.; Shang, M.; Fehlner, T. P. J. Am. Chem. Soc. 1999, 121, 1275. (24) Lei, X.; Shang, M.; Fehlner, T. P. Chem. Eur. J. 2000, 6, 2653. (25) Lei, X.; Shang, M.; Fehlner, T. P. Organometallics 2000, 19, 118. (26) Dhayal, R. S.; Chakrahari, K. K. V.; Varghese, B.; Mobin, S. M.; Ghosh, S. Inorg. Chem. 2010, 49, 7741. (27) Ghosh, S.; Lei, X.; Cahill, C. L.; Fehlner, T. P. Angew. Chem., Int. Ed. 2000, 39, 2900. (28) Fehlner, T. P. Organometallics 2000, 19, 2643. (29) Ghosh, S.; Lei, X.; Shang, M.; Fehlner, T. P. Inorg. Chem. 2000, 39, 5373. I


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