Hydrides of alo- and borogallane(4), AlGaH₄ and BGaH₄

The potential energy surfaces of the mixed gallium hydrides AlGaH₄ and BGaH₄ have been studied by ab initio molecular orbital theory at the correlated MP2 level by using a Huzinaga valence triple-ζ basis set augmented with d-polarization functions. The salt-like tridentate forms are the global minima. Their endothermic dissociation energies with respect to the neutral MH and M H₃ monomers are estimated. Hydrogenations of BGaH₄ and AlGaH₄ to the corresponding hexahydrides are calculated to be endothermic by only 2.5 and 10.8 kcal/mol. The Ga⁺[AlH₄]^- tridentate form is ca. 10 kcal/mol more stable than ionic Al⁺[GaH₄]^-. The structural and bonding similarities and differences between the aluminum- and gallium-containing systems are discussed. The ionic and covalent nature of bonding in the HF/3-21G* geometries is examined with Bader's electron density theory of atoms in molecules.