Solid-state metal hydrides provide a safe and efficient way of storing hydrogen. Those useful for applications in the energy sector, such as proton-exchange membrane (PEM) fuel cells, are mainly derived from stable intermetallic compounds and alloys. These hydrides are usually metallic and nonstoichiometric, and are called "interstitial" because hydrogen atoms occupy interstices in the metal-atom network. "Complex" metal hydrides form a distinctly different class of materials. Their name is derived from the presence of discrete metalhydrogen complexes (where the metal is a p-or d-block element) in the structures, for example, the iron-based [FeH 6 4À 18-electron complex anions, respectively. Since then, the number of such hydrides has continuously increased, and it now totals over 80 compounds containing some 30 different transition-metal hydride complexes. [1] Almost all of the known solid-state transition-metal hydride complexes are mononuclear and have terminal