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