The crystal structure of LiND2 and Mg(ND2)2

The crystal structures of LiND2 and Mg(ND2)2 have been investigated by high-resolution powder neutron and synchrotron X-ray diffraction. LiND2 takes a tetragonal crystal structure (space group I4¯, a = 5.03164(8) Å, c = 10.2560(2) Å). The Li+ ions are tetrahedrally coordinated by four amide (ND2−) ions with Li–N separations ranging between 2.065(3) and 2.210(3) Å. The two N–D distances are similar (0.967(5) and 0.978(6) Å) and the D–N–D angle is 104.0(7)°, which are very close to the isoelectronic water molecule. The neutron diffraction data for Mg(ND2)2 are fitted well with a tetragonal unit cell (space group I41/acd, a = 10.3758(6) Å, c = 20.062(1) Å), although the powder synchrotron X-ray diffraction data indicate that the symmetry may be lower. The cation is again tetrahedrally coordinated by four amide ions. The Mg–N distances are between 2.00(1) and 2.17(1) Å. The N–D separations and D–N–D angles in the three different amide ions are in the range of 0.95(1)–1.07(1) Å and 101(1)–107(2)°, respectively. The Mg(ND2)2 structure can alternatively be described as a pseudo-fcc lattice of Mg4 tetrahedra with amide anions in all tetrahedral interstices and six amide ions distributed in the periphery of each octahedral interstices, giving a slightly distorted octahedral arrangement of amide units around each Mg4 tetrahedron.