Hypoelectronicity and Chirality in Dimetallaboranes of Group 9 Metals.

The structures and energetics of the dimetallaboranes Cp2M2Bn-2Hn-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*2Rh2B10H10-n(OH)n (n = 1, 2).