Six-Vertex Hydrogen-Rich Cp₂M₂B₄H₈ Dimetallaboranes of the Second- and Third-Row Transition Metals: Effects of Skeletal Electron Count on Preferred Polyhedra

Abstract

The complete series of hydrogen-rich six-vertex cyclopentadienyl dimetallaboranes Cp 2 M 2 B 4 H 8 (Cp = η 5 -C 5 H 5 ; 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 M 2 B 4 polyhedra in the lowest energy Cp 2 M 2 B 4 H 8 structures relates to the skeletal electron count as determined by the Wade−Mingos rules. Thus, the lowest energy Cp 2 Ir 2 B 4 H 8 structures with 16 Wadean skeletal electrons have central pentagonalpyramidal Ir 2 B 4 units similar to that of the known pentagonal-pyramidal B 6 H 10 . The lowest energy Cp 2 M 2 B 4 H 8 (M = Ru, Os) structures with 14 Wadean skeletal electrons have central capped-tetragonal-pyramidal rather than octahedral M 2 B 4 units. However, isomeric Cp 2 M 2 B 4 H 8 (M = Ru, Os) structures with central M 2 B 4 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 Cp 2 M 2 B 4 H 8 structures (M = Re, Mo, W, Ta) have central M 2 B 4 bicapped tetrahedra with the metal atoms at the degree 5 vertices. Higher energy Cp 2 Re 2 B 4 H 8 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 Cp 2 M 2 B 4 H 8 (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 Cp 2 Ta 2 B 4 H 8 structure is best interpreted in having CpTa units with 16-electron rather than 18-electron tantalum configurations and a surface Ta−Ta single bond.

Combined Experimental and Theoretical Investigations of Group 6 Dimetallaboranes [(Cp*M)2B4H10] (M = Mo and W)

“…The density functional theory (DFT) calculations often play pivotal role to understand this apparent systematic rule deviation . The nature of the intermetallic interactions in the flattened dimetallaborane compounds (also termed as oblato-nido species) invoke speculations that has been dealt with several times in the literature by molecular orbital (MO) analysis. ,− The results seem to fully in agreement with some reports evidencing M–M single bond, with some others suggesting internal M–M double bond …”

The Wade–Mingos rules in seven-vertex dimetallaborane chemistry: Hydrogen-rich Cp2M2B5H9 systems of the second and third row transition metals

“…We have previously reported a density functional theory study on the six-vertex hydrogen-rich dimetallaboranes Cp 2 M 2 B 4 H 8 using the unsubstituted Cp ligand rather than the permethylated Cp* ligand to facilitate a more extensive series of computations [26]. A variety of polyhedra were found in the lowest energy structures.…”

Dimetallaborane analogues of the octaboranes of the type Cp2M2B6H10: structural variations with changes in the skeletal electron count

“…10,11 Incorporation of transition metals other than those in Group 9 (Co, Rh, Ir) into Cp 2 M 2 B 6 H 10 structures provides systems having different skeletal electron counts and thus the possibility for different geometries for their central M 2 B 6 skeletons. We have previously used density functional theory to explore this possibility with Cp 2 M 2 B n−2 H n+2 systems having central M 2 B n−2 polyhedra with five, 12 six, 13 and seven 14 vertices. The possibility for structural variation with different skeletal electron counts is illustrated with experimentally known six-vertex Cp* 2 M 2 B 4 H 8 systems having a central pentagonal pyramid for M = Ir, 15,16 a central capped tetragonal pyramid for M = Ru, 17 and a central bicapped tetrahedron for M = Re.…”

“…Ir 29 ). This paper reports a comprehensive study of the eightvertex systems Cp 2 M 2 B 6 H 10 and Cp* 2 M 2 B 6 H 10 (M = Pd, Pt, Rh, Ir, Ru, Os, Re, Mo, W, Ta) using density functional theory methods similar to those used in the previous studies [12][13][14] on the 5-, 6-, and 7-vertex systems. We studied not only systems with unsubstituted cyclopentadienyl ligands (Cp) but also those with pentamethylcyclopentadienyl ligands (Cp*) as found in the experimentally known Cp* 2 M 2 B 6 H 10 structures (M = Rh, 9 W 21 ).…”