Hypoelectronic diruthenaboranes and diosmaboranes having eight to twelve vertices: capped isocloso and bicapped closo structures

Abstract

The oblate deltahedral structures for the experimentally known 2n − 4 Wadean skeletal electron systems Cp₂Re₂B_n−2H_n−2 (Cp = η⁵-R₅C₅ ligand; R = H, CH₃; n = 8 to 12) differ radically from the more nearly spherical 2n skeletal electron isocloso deltahedra such as the known Cp₂Fe₂C₂B_n−4H_n−2 and 2n + 2 skeletal electron closo systems such as the extensive series of known Cp₂Co₂C₂B_n−4H_n−2 derivatives. Density functional theory has now been used to investigate the intermediate “missing” systems, namely the 2n − 2 Wadean skeletal electron systems Cp₂M₂B_n−2H_n−2 (M = Ru, Os). The lowest energy such structures are predicted to have central deltahedra with n − 1 or n − 2 vertices capped by the remaining BH vertices. This generates larger deltahedra having one or two degree 3 vertices leading to tetrahedral cavities. In all of the lowest energy Cp₂M₂B_n−2H_n−2 structures the metal atoms are located at the highest degree vertices, most frequently degree 6 vertices. These metal vertices share a deltahedral edge with M–M distances ranging from 2.7 to 2.9 Å and Wiberg bond indices of ∼0.3 to ∼0.4. The structural pattern changes drastically for the 12-vertex systems Cp₂M₂B₁₀H₁₀ (M = Ru, Os) where the lowest energy structures are “isoisocloso” deltahedra having two adjacent degree 6 vertices for the metal atoms as well as eight degree 5 and two degree 4 vertices. Higher energy Cp₂M₂B₁₀H₁₀ structures are based on a regular icosahedron with all degree 5 vertices having unusually short MM edges of ∼2.2 to ∼2.3 Å with corresponding high Wiberg bond indices of ∼1.3 to ∼1.5 suggesting formal metal–metal triple bonds.