Complex Transition-Metal Hydrides

Abstract

Complex transition-metal hydrides provide new opportunities for hydrogen 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 hydrocarbons (methane: H/C = 4); their hydrogen-volume efficiencies exceed that of liquid hydrogen by a factor of up to two (Mg₂FeH₆), their weight efficiencies exceed 5% (Mg₃MnH₇), and their hydrogen dissociation temperatures under 1 bar hydrogen pressure range from ca. 100° (NaKReH₉) to 400° (CaMgNiH₄). Their crystal chemistry is extremely rich and shows a large inventory of transition-metal hydrido complexes that often conform to the 18-electron rule. New synthetic methods are likely to yield further members of this class of materials.