Effects of extra hydrogens on the electronic structures of five- and six-vertexed polyhedral boranes

Abstract

The effects on the electronic structure of [B₅H₅]^2– and [B₆H₆]^2– of adding one or two extra hydrogens in terminal and bridging positions have been studied by extended-Hückel molecular-orbital calculations. Trigonal-bipyramidal and square-pyramidal [B₅H₆]^– and B₅H₇ are the five-vertexed species considered; the six-vertexed ones are octahedral, trigonal-prismatic, and capped-square-pyramidal [B₆H₇]^– and B₆H₈. Terminal and edge-bridging sites are preferred for [B₅H₆]^– and [B₆H₇]^–(octahedral) respectively. For polyhedra consisting entirely of triangulated faces there is little site preference for the bridging ligands. However, when a square face is present, there is a marked preference for a bridging ligand to be on an edge of that face. This effect is so marked that the presence of these extra atoms could alter the favoured cluster framework from that of the polyhedral anion to one having a tetragonal face. Generally, adding the extra ligands does not alter the number of bonding orbitals from that in the polyhedral dianion.