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

Abstract

The structures and energetics of the complete series of hydrogen-rich dimetallaboranes Cp₂M₂B₆H₁₀ and Cp*₂M₂B₆H₁₀ (Cp = η⁵-C₅H₅; Cp* = η⁵-Me₅C₅; M = Pd, Pt; Rh, Ir; Ru, Os; Re; Mo, W; Ta), including the experimentally known Cp*₂Rh₂B₆H₁₀ and Cp*₂W₂B₆H₁₀ (Cp* = η⁵-Me₅C₅), have been investigated by density functional theory. The lowest energy structures of the hyperelectronic Cp₂M₂B₆H₁₀ (M = Pd, Pt; Rh, Ir) systems have central M₂B₆ frameworks with a hexagonal open face similar to the B₈ networks in arachno-B₈H₁₄ and nido-B₈H₁₂. The two lowest energy structures for Cp₂Rh₂B₆H₁₀ and Cp*₂Rh₂B₆H₁₀, lying within 1 kcal mol⁻¹ of energy, differ only in the locations of the bridging hydrogen atoms around the hexagonal hole consistent with the experimentally observed fluxionality of the hydrogen atoms in Cp*₂Rh₂B₆H₁₀. Most of the lowest energy Cp₂M₂B₆H₁₀ (M = Ru, Os) structures also have a central M₂B₆ framework similar to B₈H₁₂, typically with such additional features as an additional metal–metal bond or a formal metal–metal double bond. A common motif for the low-energy structures of the hypoelectronic Cp₂M₂B₆H₁₀ (M = Re; Mo, W; Ta) systems, including the experimentally known Cp*₂W₂B₆H₁₀, is a central M₂B₄ octahedron with its two M₂B faces capped by the remaining boron atoms and with four M–B edges bridged by hydrogen atoms. Such structures can also be considered as oblatonido structures derived from the experimentally known 9-vertex oblatocloso Cp*₂Re₂B₇H₇ structure by removal of the unique degree 4 vertex atom.