Unusual and Conventional Dative Bond Formation by s Lone Pair Donation from Alkaline Earth Metal Atoms to BH, AlH, and GaH.

Using ab initio electronic structure methods with flexible atomic orbital basis sets, we examined the nature of the bonding arising from donation of an ns electron pair on an alkaline earth atom (Mg or Ca) into a vacant n'p orbital on the group 13 atom of BH, AlH, or GaH. We also examined what happens when an excess electron is attached to form corresponding molecular anions. Although the geometries of MgBH, MgAlH, MgGaH, and CaBH are found to be much as one would expect for datively bound molecules, CaAlH and CaGaH were found to have very unusual geometries in that their Al-H or Ga-H bonds are directed toward the Ca atom rather than away, as in the other compounds. Internal electrostatic Coulomb attractions between the partially positively charged Ca center and the partially negatively charged H centers were suggested as a source of these unusual geometries. The other novel finding is that the electron affinities (EAs) of all six M'-MH species lie in the 0.7-1.0 eV range, which is suggestive of ionic electronic structures for the neutrals even though the partial charges on the alkaline earth centers are as low as 0.3 atomic units. Partial positive charge on the alkaline earth atoms combined with substantial electron affinities of the BH, AlH, and GaH groups, but only when distorted from planar geometries, were suggested to be the primary contributors to the large EAs.