The structures and energetics of the dirnetallaboranes Cp2M2Bn-2Hn-2 (n = 8, 9, 10, 11, 12; M = Co, Rh, Ir; Cp =eta(5)-C5H5) were studied using density functional theory. The lowest energy Cp2M2B6H6 and Cp2M2B7H7 structures are chiral C-2 structures based on the corresponding closo deltahedra, namely the bisdisphenoid and the tricapped trigonal prism. The permethylated iridaboranes Cp*2Ir2B6H6 and Cp*2Ir2B7H7 (Cp* = eta(5)-Me5C5) were synthesized by Ghosh and co-workers. However, they were found by Xray 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/isodoso 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).