A molecular orbital treatment of diborane as a four-centre, four-electron problem

Abstract

The Roothaan self-consistent field equations in their linear combination of atomic orbitals form have been solved for a system composed of four electrons in the field of a framework of two asymmetric boron cores of net charge +1 each and two protons in the 'bridge' positions as in diborane. Exact values for all one and two-centre integrals were used, and approximations were made for three- and four-centre integrals. The molecular orbitals of lowest energy are the completely symmetric orbital and the π -orbital with a nodal plane normal to the line between the protons. In terms of the molecular-orbital coefficients, there is a small residual net negative charge on the hydrogens, i. e. the bridge hydrogens have a hydridic character. In terms of configurational expansions, a localized three-centre bond description of the molecule is very close to the actual configuration and is somewhat better than a description as a resonance hybrid between either ordinary covalent valence bond structures or localized two-centre molecular orbital structures.