Effects of Orbital Vacancies in Boron Compounds

Addition of H2to BH3 yields BE5, which is shown to be of Cq 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 exammed 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 donatiôn becomes important, where fractional bonds occur, and where a more complex description can usefully replace a large number of valence structures. Vacant orbital contributions 1ave also been used to guess where ligand. (e.g. H) attack may occur, and at. which pair of adjacent boron atoms BE3 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 BE3 with higher hydrides, and of higher hydrides with each other may be expected .to follow this principle. I. VACANT ORBITALS IN TRANSIENT SPECIES BE A prototype of the BE units in polyhedral itolecules, the diatomic BE molecule has all of its electrons paired (-E+). Nevertheless, it is paramagnetic. The magnetic susceptibility, x = 18.7 ppm (1), is the temperature-independent type, and arises largely from excitations from the filled 3a level. This unusual property is a forewarning that magnetic shielding in the boron hydrides and carboranes have large, sometimes dominating, contributions from the paramagnetic term. The chemistry of this unstable species has not been studied, but would probably be very interesting. BE3 iiwn as a reaction intermediate in diborane pyrolysis and other reactions, this molecule has a vacant orbital which cannot be filled by a molecular distortion. Adducts to Lewis bases have stabilities correlating with bond distance (2). It has been isolated in a matrix (3) atlow temperatures, from pyrolysis of BH3CO. Theoretical studies are consistent with the planar D3h structure. We now turn to a very weak adduct between BH3 and H2.