Dimetallaborane analogues of pentaborane

Abstract

The structures of five-vertex dimetallaboranes Cp₂M₂B₃H₇ (Cp = η⁵-C₅H₅) of the second and third row transition metals, including the experimentally known Cp*₂Rh₂B₃H₇ (Cp* = η⁵-Me₅C₅), have been investigated by density functional theory. The predicted low-energy structures for Cp₂M₂B₃H₇ (M = Rh, Ir) are tetragonal pyramids similar to Cp*₂Rh₂B₃H₇ and pentaborane-9 B₅H₉ and consistent with their 14 Wadean skeletal electrons. Two Cp*₂Rh₂B₃H₇ structures with the same central Rh₂B₃ tetragonal prism are found with energies within ∼1 kcal mol⁻¹ of each other, consistent with the experimental observation of two isomers in solution. The electron-richer Cp₂M₂B₃H₇ (M = Pd, Pt) systems having 16 Wadean skeletal electrons are predicted to exhibit more open structures analogous to the known structure for the valence isoelectronic pentaborane-11 B₅H₁₁. Trigonal bipyramids with the metal atoms at equatorial vertices are typically found to be low-energy structures for the hypoelectronic Cp₂M₂B₃H₇ systems (M = Ru, Os, Re, Mo, W, Ta). In addition, the low-energy Cp₂Re₂B₃H₇ structures of the rhenium derivatives Cp₂Re₂B₃H₇ provide examples of structures based on a central Re₂B₂ tetrahedron with the Re–Re edge bridged by the third boron atom. Such structures can be derived from a trigonal bipyramid by the rupture of one of the axial–equatorial edges.