Our interest in materials for hydrogen storage applications has prompted us to investigate the structure of complex metal hydrides.' The ideal storage material would have a very high hydrogen density. Of the known complex metal hydrides, dipotassium nonahydridorhenate, KzReH? has a hydrogen density of 103 g of WL, approximately 1.5 times that of liquid hydrogen. It also has a 3:l hydrogen to metal atom ratio, one of the highest known. The [ReH& anion has tricapped trigonal prismatic symmetry (D3h).3 In the revised synthesis of this hydride? the disodium salt is first synthesized and then converted to the dipotassium salt by way of the barium salt. This intermediate, BaReH9, has no reported characterizations, however the hydrogen-to-metal ratio is 4.5:1, making it the metal hydride with the highest known hydrogen to metal ratio. With the higher hydrogen-to-metal ratio, the hydrogen density might also be expected to be increased. For this reason, we have undertaken an investigation of this compound.Barium nonahydridorhenate was prepared according to the published procedure from the disodium salt.4 The conversion gave average yields of approximately 95%; overall yields of approximateIy 40% were obtained based on the initial perrhenate. The dried white powder slowly turned gray and eventually black when exposed to air for periods of several weeks. X-ray powder diffraction and IR measurements of the colored powder after prolonged exposures showed the presence of the hydride with an additional strong peak at 1450 cm-' but no perrhenate absorptions. The frequency of the new absorption corresponds to the frequency normally attributed to carbonate impurities.2 The decomposition of this salt therefore appears to proceed via a pathway different from that of the alkali metal salts.4bAn IR spectrum of the dried white powder, in KBr pellets, was measured on a Perkin-Elmer 883 spectrophotometer. An
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