Bonding in Boron Hydrides

Unique preferred localized valence structures for B2H6, B4H10, B5H,, and B6H10 have been found from accurate self-consistent field wavefunctions by maximizing Coulomb repulsions of electron pairs within orbitals. Objective evidence has thus been obtained for two-centre BH, three-centre bridge BHB, two-centre BB and central three-centre BBB bonds, but not, as yet, for open three-centre BBB bonds. Locali7ation in B5H9 is ambiguous, as it is in organic molecules such as benzene. More thermodynamical stable isomers or plausible reaction pathways appear to have more resonance structures, and less reactive (or more plausible) structures or intermediates appear to have more nearly uniform charge distributions. INTRODUCTION Given the same number of valence orbitals as carbon but one less electron, boron is said to form electron-deficient compounds. However, in another sense these compounds are not deficient because the extended topologies of carbon chemistry are not found in boron compounds. Instead, a contracted topology occurs, mainly exemplified geometrically by bridge hydrogens and by boron triangles. The simplest valence-theoretical description of this contracted topology is the three-centre two-electron bond, which has gradually developed into a general theory' of bonding in boron hydrides, their derivatives and their plausible reaction intermediates. Only among the simplest structures are unique, or preferred, valence descriptions possible. More complex structures require resonance descriptions when three-centre and two-centre bonds are used as a basis6. Nevertheless, in the sense that the most stable orbitals are filled, and that a substantial energy gap exists towards the unfilled (excited) state, these electron-deficient' molecules are indeed closed-shell species. Molecular orbital theory in the form of the extended Hückel theory was originated and first applied to these larger boron hydrides6'7 in order to provide an alternative to these resonance descriptions. Subsequently, both the logical basis and the source of parameters for these molecular orbital studies were greatly improved, and relationships were thereby established8'9 to the rigorous molecular self-consistent field (SCF) method' .By the averaging of Coulomb repulsions, but the inclusion of exchange interactions and use of