Heptagonal bipyramid and flattened oblatocloso niobium and tantalum dicarbaborane structures.

The structures and energetics of the diniobadicarbaboranes and ditantaladicarbaboranes Cp2M2C2Bn-4Hn-2 (Cp = η5-C5H5; M = Nb, Ta; n = 8 to 12) have been examined by density functional theory methods. The low-energy structures include flattened oblatocloso structures related to those of the experimentally known dirhenaboranes Cp*2Re2Bn-2Hn-2 (Cp* = η5-Me5C5) with two more skeletal electrons. In addition, a few closo and isocloso deltahedral structures are found having adjacent metal atoms with short metal-metal distances suggesting substantial multiple-bond character, with formal bond orders of at least three according to electron counting. The 9-vertex Cp2M2C2B5H7 (M = Nb, Ta) systems are particularly interesting since heptagonal bipyramidal structures with degree 7 metal vertices as well as oblatocloso(6,6) structures with degree 6 metal vertices have similar energies but with intradeltahedral metal-metal distances differing by ∼0.4 Å indicating widely different levels of flattening. The low-energy structures of the 8-vertex Cp2M2C2B4H6 systems are all hexagonal bipyramids with varying locations of the two carbon atoms on the equatorial hexagon. The low-energy structures of the 12-vertex Cp2M2C2B8H10 systems include oblatocloso deltahedra with degree 7 and 6 metal vertices as well as a deltahedron with two degree 6 metal vertices, two degree 4 vertices, and eight degree 5 vertices. However, no low-energy 12-vertex structures were found based on a regular icosahedron with exclusively degree 5 vertices.