THEORETICAL STRUCTURES OF ALUMINUM BORON HYDRIDES

The ab initio structures of AlBHn, n = 1 to 6, have been investigated at the HF/6-31G*//HF/6-31G* level. The effects of correlation using Møller-Plesset perturbation theory at the MP4SDTQ//HF/6-31G* level have been included. For all the hydrides, the structures are determined primarily by the fact that B-H bonds are significantly stronger than Al-H bonds. In the case of AlBH3, the lowest energy isomer is a Lewis adduct between an Al atom and BH3 molecule, suggesting the application of BH3 as an atom trap. For AlBH4, the lowest energy confomer is an ionic structure, Al+BH4-. The double H-bridged radical, H2Al(H2)BH, is more stable than the single H-bridge radical, H2Al(H)BH2, contrary to that found for the diboryl radical. Similarly, H2Al(H2)AlH is more stable than H2Al(H)AlH2. This reversal in isomer stability is discussed in terms of a Lewis acid/base interaction between XH2 radicals and XH3, where X = B, Al. The hexahydrides, dialane(6) and aluminoborane, AlBH6, have structures similar to that of diborane. © 1990 American Chemical Society.