Mg3MnH7, containing the first known hexahydridomanganese(I) complex

Mg3MnH7 was synthesised by reacting MgH2 with manganese powder in a 3:1 molar ratio at 20 kbar and 1070 K for 120 min, and investigated by neutron powder diffraction on its deuteride. It crystallises with the hexagonal Mg3ReH7 structure type (space group P6(3) Immc, Z=2, a=4.7099(1), c=10.2861(3) Angstrom (hydride); R=4.6998(1), c=10.2590(2) Angstrom (deuteride). Manganese is nearly octahedrally co-ordinated by deuterium with bond distances of 1.629(3) Angstrom and bond angles of 89.17(14) and 90.83(2)degrees. An additional deuteride-anion is co-ordinated by five magnesium cations in a trigonal pyramidal way. Mg3MnH7 decomposes between 530 K and 640 K under 1 bar hydrogen pressure into elemental manganese and magnesium. Its hydrogen storage efficiency is 5.2 wt % or 118.5 g H-2 I-1. (C) 1998 Elsevier Science S.A. All rights reserved.