LaMg2NiH7, a novel quaternary metal hydride containing tetrahedral [NiH4]4- 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) Angstrom; V=364.0(1) Angstrom(3); Z=4) and absorbs hydrogen near ambient conditions (<200degreesC, <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 P2(1)/c, a=13.9789(7), b=4.7026(2), c=16.0251(8) Angstrom; beta=125.240(3)degrees, V=860.39(8) Angstrom(3); Z=8) that contains two symmetry independent tetrahedral [NiD4](4-) Complexes with Ni-D bond lengths in the range 1.49-1.64 Angstrom, and six D- anions in tetrahedral metal configuration with bond distances in the ranges 1.82-2.65 Angstrom (Mg) and 2.33-2.59 Angstrom (La). The compound constitutes a link between metallic 'interstitial' hydrides and non-metallic 'complex' metal hydrides. (C) 2002 Elsevier Science B.V. All rights reserved.