Structure and electron counting in ternary transition metal hydrides

A large number of ternary hydrides of transition metals and alkali or alkaline earth metals have been synthesized and structurally characterized in the last twenty years. These compounds exhibit a puzzling variety of compositions, transition metal coordination numbers, transition metal coordination geometries, and distribution of hydrides within and outside of the transition metal coordination sphere. Valence Bond (VB) concepts form a theoretical framework for understanding, at least partially, some of the dominant trends observed among various transition metal hydride structures. Extrapolation of these concepts suggests that synthesis of ternary metal hydrides with formal electron counts at the transition metal exceeding 18 electrons may be feasible.