Lewis base interaction with gallium hydrides: a computational study

The recent synthesis and structural characterization of the complex of 3,5-dimethyl-4-hydropyridyl-gallane 1 with the Lewis base 3,5-dimethylpyridine revealed an unusually large angle α = H-Ga-H, 127(2)°, at variance with expected steric effects of the bulky substituents at the tetrahedrally coordinated Ga center. This finding prompted us to study computationally gallium hydrides using density functional and post-Hartree-Fock methods. For 1, we estimated α at 131° from a calculation on 4-hydropyridyl-gallane, GaH₂(Hpy). This value is reduced by 3° due to the interaction with Lewis base pyridine, to yield α = 128°, in excellent agreement with experiment. With an analysis of orbital interactions and a natural bond orbital analysis, we rationalized structural variations of donor-acceptor adducts LGaH₂X where X is a substituent and L is a Lewis base. Angle α is mainly determined by the polarity of the Ga-X bond: the more electronegative substituent X, the larger α and the stronger the interaction of GaH₂X with L. Interaction with a weak base L slightly distorts the initially planar geometry of the dihydride to a trigonal pyramidal form; for a strong base, the structure can become pseudo-tetrahedral.