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).