Coexistence of hydrogen and polyanions in multinary main group element hydrides

Multinary hydrides based on mixtures of s- and p-block metals are often not fully hydrogenated, but possess a polymeric anion which is formed by the p-block component upon reduction by the s-block metals. Hydrogen may occur hydridic and exclusively coordinated by s-block metal ions (Zintl phase hydrides), or be incorporated in the polymeric anion where it acts as a covalently bonded terminating ligand (polyanionic hydrides). The current state of Zintl phase hydrides and polyanionic hydrides is reviewed with an emphasis on crystal structures and bonding principles.