LaMg2NiH7, a novel quaternary metal hydride containing tetrahedral [NiH4] complexes and hydride anions

A new ternary compound of composition LaMg₂Ni has been found and investigated with respect to structure and hydrogenation properties. It crystallizes with the orthorhombic MgAl₂Cu type structure (space group Cmcm, a=4.2266(6), b=10.303(1), c=8.360(1) Å; V=364.0(1) ų; Z=4) and absorbs hydrogen near ambient conditions (<200 °C, <8 bar) thereby forming the quaternary metal hydride LaMg₂NiH₇. Neutron powder diffraction on the deuteride revealed a monoclinic distorted metal atom substructure (LaMg₂NiD₇: space group P2₁/c, a=13.9789(7), b=4.7026(2), c=16.0251(8) Å; β=125.240(3)°, V=860.39(8) ų; Z=8) that contains two symmetry independent tetrahedral NiD₄]^4? complexes with Ni–D bond lengths in the range 1.49–1.64 Å, and six D^?anions in tetrahedral metal configuration with bond distances in the ranges 1.82–2.65 Å (Mg) and 2.33–2.59 Å (La). The compound constitutes a link between metallic ‘interstitial’ hydrides and non-metallic ‘complex’ metal hydrides.