Lewis acid-Lewis base addition compounds of boron-containing molecules and their charge transfer complex behaviour

Abstract

Delocalized electron-deficient molecules such as the higher boron hydrides appear to act as Lewis acids or bases with properties formally analogous to those of delocalized π-electron systems as well as by the customary behaviour of vacant orbital acceptance or onium-type donation. On this basis, the behaviour of ZB 10H 12Z and ZB 9H 13 complexes (when Z is a neutral molecule, even-electron donor) can be described in terms of charge transfer complex theory. ZB 10H 12Z and ZB 9H 13 compounds form an almost unique class of compounds in that they combine features of both inter-molecular and intramolecular charge transfer complexes. The long wave length transitions in PyB 10H 12Py complexes (Py = pyridine) are shown to be dependent upon the energies of the π ∗ lowest unfilled molecular orbitals (LUMO) of the substituted pyridines as calculated by MO-LCAO methods (hence the transitions are confirmed as B 10H 12 → Py). From the differences in the wave lengths of analogous bands, the energy of the highest occupied molecular orbital (HOMO) of B 10H 12 (modified by attachment to 2 Py) above the energy of B 9H 13 (modified by attachment to a Py) may be evaluated. If reasonable numerical substitutions for α, β and γ are made in the energy expression for the π ∗ LUMO the energies of the HOMO of B 10H 12 and B 9H 13 below the zero of the energy scale can be estimated. For effective ZB 10H 12Z formation it is concluded Z must not only be a good n-donor but should also be a good π- (or d-) acceptor. Stabilities of ZB 10H 12Z compounds Py > ArNR 2 are opposite to those of BY 3·Z compounds R 3N > ArNR 2 > Py, whereas rates of formation reactions are proportional to base strengths of the donors.