AN ORBITAL ANALYSIS OF HYDROGEN PAIRING IN NONSTOICHIOMETRIC TRANSITION-METAL HYDRIDES

Hydrogen-hydrogen pairing is explored in the low hydrogen concentration metal hydride YH0.2, using extended Huckel tight-binding calculations. A comparison with H-2 molecular complexes, where close H-H contacts occur, allows us to explain why through-space pairing must be ruled out in YH0.2. An orbital explanation of the Switendick criterion is given. Compared to various possible occupation sites, the through-atom coupling observed in YH0.2 might be favored because of Peierls distortion effects. This preference appears to be sensitive to the nature of the metallic host and the hydrogen concentration.