Synthesis and structure determination of complex zinc hydrides .4. Trirubidium and tricaesium tetrahydridozincate(II) hydride, Rb3ZnH5 and Cs3ZnH5

Rb3ZnH5, Cs3ZnH5 and their deuterides have been prepared by reacting stoichiometric mixtures of the binary alkali hydrides (deuterides) and zinc metal powder at 650 K under 80 bar hydrogen (deuterium) pressure, and characterized by X-ray and neutron powder diffraction at room temperature. The compounds crystallize with the Cs3CoCl5 structure type, space group I4/mcm, Z=4, and contain tetrahedral [ZnD4](2-) complexes with Zn-D bond distances of 1.659(2) Angstrom (Rb) and 1.704(3) Angstrom (Cs), and octahedrally coordinated hydride anions with M-D average distances of 2.91 Angstrom (Rb) and 3.05 Angstrom (Cs). The displacement amplitudes of hydrogen indicate the onset of orientational disorder of the tetrahedral complexes. The compounds decompose at 630 K (Rb) and 625 K (Cs) in hydro en at ambient pressure. Their hydrogen storage efficiencies are of 45.5 (Rb) and 39.7 g H-2 l(-1) (Cs), and 1.53 (Rb) and 1.1 wt% (Cs).