Abstract
Solid-state metal hydrides display hydrogen densities close to that of liquid hydrogen and thus provide a safe and efficient way of storing hydrogen. As a result of recent neutron and synchrotron diffraction work some novel metal hydrides have been characterized that shed new light on the nature of metal-hydrogen interactions. While hydrogen appears as an anion surrounded by a large inventory of cation configurations in ionic hydrides such as Ca~4~Mg~3~H~14~, Ca~19~Mg~8~H~54~, Eu~2~MgD~6~, Eu~6~Mg~7~D~26~ and Eu~2~Mg~3~D~10~, it acts as a terminal ligand in covalently bonded hydride complexes based on p-elements such as [BH~4~]^β^ and d-elements such as [IrH~5~]^4β^ and [IrH~4~]^5β^ in the complex hydrides LiBH~4~ and Mg~6~Ir~2~H~11~, respectively. Surprisingly, hydride complexes and hydride anions can also be discerned in typically metallic (interstitial) hydrides such as NdMgNi~4~H~4~ (= Nd^3+^Mg^+2^Β·[Ni~4~H~4~]^5β^) and LaMg~2~NiD~7~ (= La^3+^Mg^+2^
~2~Β·[NiH~4~]^4β^Β·3H^β^). Some hydrides also reveal other interesting features such as a hydrogenation induced Ce^4+^ β Ce^3+^ valence change in CeMn~1.8~Al~0.2~H~4.4~ at room temperature that is accompanied by a Mn/Al metal atom exchange over distances of ~2.6 Γ
, and a hydrogen induced metal-to-nonmetal transition near ambient conditions that leads from the metallic compound Mg~3~Ir to the red colored hydride Mg~6~Ir~2~H~11~. In this article recent work and some methodological aspects are highlighted.