Complex Transition Metal Hydrides

Compounds having structural units formed by transition metals and hydrogen have only become known fairly recently. Only the synthesis and structural characterization of ternary metal hydrides AxMyHz in which A is an alkali or alkaline‐earth metal and M is a transition metal, led to the discovery of anionic complex groups of the form [MHz]. Although there are obvious structural similarities to the corresponding halides and oxides, the dynamic properties of the hydride ligand are somewhat unusual and are responsible for numerous phase transformations. Transitions throughout the range from saltlike to metallic behavior make us expect interesting physical properties and applications. From metallic to saltlike behavior is observed for the ternary metal hydrides AxMyHz (A = alkali or alkaline‐earth metal, M = transition metal), which contain discrete anionic units of the type [MHx]n−. The mobility of the hydrogen ligands leads to a remarkable phase behavior, which may find technical application. For instance, in K3PdH3 the cubic modification at high temperature (right: Pd atoms of the [PdH2]2− units and the hydrido‐centered octahedrons of K atoms) is transformed below 500 K into a tetragonal form in which the K octahedrons are tilted away from each other.