Unusual dimetallaborane cluster polyhedra and their skeletal bonding

A variety of stable polyhedral dimetallaboranes have been synthesized exhibiting unusual polyhedral structures that challenge concepts of chemical bonding. In order to understand the underlying basis for such structures for dimetallaboranes and related species, comprehensive density functional studies have been performed on many such systems. This paper reviews the highlights of our recent work in this area. Systems discussed include the following: (1) The systems Cp2Ni2Bn−2Hn−2 (Cp = η5-C5H5) and Cp2M2C2Bn−4Hn−2 (M = Co, Rh, Ir) having 2n + 2 Wadean skeletal electrons and exhibiting most spherical closo deltahedral structures; (2) The slightly hypoelectronic systems Cp2M2Bn−2Hn−2 (M = Co, Rh, Ir) and Cp2Fe2C2Bn−4Hn−2 having only 2n Wadean skeletal electrons and in some cases, particularly the 10-vertex systems, exhibiting structures based on alternative isocloso deltahedra providing a degree 6 vertex for a metal atom; (3) Even more hypoelectronic dirhenaboranes and trirhenaboranes including flattened deltahedral oblatocloso structures exhibiting internal Re-Re bonding as well as closo and isocloso structures with short Re-Re edges suggesting formal triple and quadruple bonds; (4) Other hypoelectronic dimetallaboranes Cp2M2Bn−2Hn−2 with 2n − 2 Wadean skeletal electrons (M = Ru, Os) and 2n − 6 Wadean skeletal electrons (M = Mo, W); (5) Hydrogen-rich dimetallaboranes Cp2M2Bn−2Hn+2 (M = Ta, Mo, W, Re, Ru, Os, Rh, Ir, Pd, Pt; n = 5, 6, 7, 8), typically having the four “extra” hydrogen atoms bridging edges of an open polyhedral face.