Effects of orbital vacancies in boron compounds

Abstract

Abstract Addition of H2 to BH3 yields BH5, which is shown to be of Cs symmetry. Rearrangement mechanisms and hydrogen loss have been examined. Intermediates such as B3H7 and B4H8 have been predicted to have vacant orbital structures in which the stability gained by filling that orbital is balanced by strain in converting a terminal hydrogen to a bridge hydrogen. Other factors, such as hyperconjugative interactions and boron framework distortions are of importance. These factors have been examined also in bridge hydrogen asymmetries and distortions from regular geometry, in the light of vacant orbital contributions to valence structures of known boranes and carboranes. Localization procedures have refined the concepts of where an open three-center bond is useful, where single bond donation becomes important, where fractional bonds occur, and where a more complex description can usefully replace a large number of valence structures. Vacant orbital contributions have also been used to guess where ligand (e.g. H) attack may occur, and at which pair of adjacent boron atoms BH3 may be expected to add. A study of diborane formation indicates favoritism for simultaneous H…B and B…H reaction of compounds of the two distinct BH3 reactants. Reactions of BH3 with higher hydrides, and of higher hydrides with each other may be expected to follow this principle.