Metal Hydrides: Transition Metal Hydride Complexes

Solid-state metal hydrides provide a safe and efficient way of storing hydrogen (Schlapbach 2002). They are commercialized for use in rechargeable batteries and developed for energy conversion devices such as fuel cells (see Hydrogen Metal Systems: Electrochemical Reactions). A great majority of metal hydrides derives from intermetallic compounds and alloys (see Hydrogen Metal Systems: Hydride Forming Alloys). They have metallic properties, are nonstoichiometric, and are usually called ‘‘interstitial’’ because hydrogen occupies interstices in the metal atom network (Yvon and Fischer 1988). Due to their high mobility the hydrogen atoms are usually disordered and may enter and quit the metal matrix near ambient conditions. ‘‘Complex’’ metal hydrides form a subclass of metal hydrides. They derive their name from the presence of discrete metal–hydrogen complexes in the crystal structure that are centered by dor p-elements. The compounds are stoichiometric and usually nonmetallic, and the hydrogen ligands are usually ordered at ambient conditions. Consequently, hydrogen in these compounds is less mobile and leaves less readily the metal atom network. Among the hydrides based on d-elements, the historically first member and textbook example is K2ReH9. It was reported in 1964 and found to contain tricapped trigonal prismatic [ReH9] 2 complexes. Since then the number of complex metal hydrides has continuously increased and now totals over 80 compounds containing some 30 different metal hydride complexes. They have been reviewed at various time intervals (Bronger 1991, Yvon 1994, 1998, Bau 1997, King 2000) and incorporated into a public online database on metal hydrides (Sandrock and Thomas 1997). The present update covers the currently known solid-state complex transition metal hydrides and some of their properties of interest for science and technology. Both mononuclear (terminal hydrogen ligands) and polynuclear (bridging hydrogen ligands) complexes are covered. Furthermore, some recently reported examples for hydrogenationinduced complex formation and metal–insulator transitions in typically metallic (interstitial) hydrides are also included.