A molecular-orbital evaluation of skeletal electron-counting procedures

Abstract

Extended-Hückel molecular-orbital calculations have been used to follow the changes in electronic structure which occur when idealised borane polyhedra are synthesised from each other by the capping of faces and edges with single BH units or by the mutual approach of two faces. Structures derived from a polygon by face capping maintain the basic electronic structure of the polygon. However, the outcome of the approach of two triangular faces is dependent on their relative orientations. Energies of alternative structures of six-, seven-, and eight-vertexed structures have been calculated for different overall charges. A different structure is favoured for [B₆H₆]ⁿ⁺ where n= 0, –2, –4, and –6 (bicapped-tetrahedral, octahedral, pentagonal-pyramidal, and trigonal-prismatic respectively). All the stable forms have their bonding orbitals filled and, on that basis, the structures are anticipated by the capping principle. However, eight- and seven-vertexed structures are dominated by dodecahedral and nido-dodecahedral forms respectively. Rigorous application of the debor and capping principles of the conservation of the number of skeletal bonding orbitals may be misleading in some cases.