COMPLEX TRANSITION-METAL HYDRIDES

Complex transition-metal hydrides provide new opportunities for hydrog en storage. Their hydrogen-to-metal ratios reach values of up to H/M = 4.5 (BaReH9) and thus surpass the hydrogen-to-carbon ratios of hydroc arbons (methane: H/C = 4); their hydrogen-volume efficiencies exceed t hat of liquid hydrogen by a factor of up to two (Mg2FeH6), their weigh t efficiencies exceed 5% (Mg3MnH7), and their hydrogen dissociation te mperatures under 1 bar hydrogen pressure range from ca. 100 degrees (N aKReH9) to 400 degrees (CaMgNiH4). Their crystal chemistry is extremel y rich and shows a large inventory of transition-metal hydride complex es that often conform to the 18-electron rule. New synthetic methods a re likely to yield further members of this class of materials.