Effects of Orbital Vacancies in Boron Compounds

Abstract

-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. I. VACANT ORBITALS IN TRANSIENT SPECIES BEA 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. BE3iiwn 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. . When BH4D is produced by reaction (4) .of BE4 with D30 below pH 12, about 95% of the hydrogen loss occurs as ED. This interesting observation implies either (a) that the D+ becomes equivalent to only one resident H atom, or (b) that the D remains unique and is lost when BH4D decomposes (A candidate for this second possibility is that D remains at the apex of tetragonal 701