Hypoelectronicity and Chirality in Dimetallaboranes of Group 9 Metals

The structures and energetics of the dimetallaboranes Cp₂M₂B_n-2H_n-2 (n = 8, 9, 10, 11, 12; M = Co, Rh, Ir; Cp = η⁵-C₅H₅) were studied using density functional theory. The lowest energy Cp₂M₂B₆H₆ and Cp₂M₂B₇H₇ structures are chiral C₂ structures based on the corresponding closo deltahedra, namely the bisdisphenoid and the tricapped trigonal prism. The permethylated iridaboranes Cp*₂Ir₂B₆H₆ and Cp*₂Ir₂B₇H₇ (Cp* = η⁵-Me₅C₅) were synthesized by Ghosh and co-workers. However, they were found by X-ray crystallography to have nondeltahedral structures containing a quadrilateral face, namely a bicapped trigonal prism and a capped square antiprism for the 8- and 9-vertex systems, respectively. These structures correspond to a mean of the two opposite enantiomers and can also represent the "square" intermediate in the interconversion of the two enantiomers. The lowest energy structures for the 10-vertex Cp₂M₂B₈H₈ systems are two isocloso deltahedra with one metal atom at a degree 6 vertex and the other metal atom at a degree 5 vertex. Both isomers have been realized experimentally for Cp₂Ir₂B₈H₈. The lowest energy structures for the 11-vertex Cp₂M₂B₉H₉ systems have central closo/isocloso deltahedra with one metal atom at a degree 6 vertex and the other metal atom at a nonadjacent degree 5 vertex. This structure type has been found experimentally in both the rhodaboranes and iridaboranes Cp*₂M₂B₉H₉ (M = Rh, Ir). The lowest energy structures for the 12-vertex systems Cp₂M₂B₁₀H₁₀ (M = Co, Rh, Ir) are deltahedra with two adjacent degree 6 vertices for the metal atoms. This type of structure is found experimentally in the rhodium complexes Cp*₂Rh₂B₁₀H_10-n(OH)_n (n = 1, 2).