Synthesis and crystal structure of (NH)[Ni(HAsO)(AsO)(OH)].

The title compound, ammonium trinickel(II) hydrogen arsenate arsenate di-hydroxide, was synthesized under hydro-thermal conditions. Its crystal structure is isotypic with that of K[Cu(HAsO)(AsO)(OH)] and is characterized by pseudo-hexa-gonal (001) [NiAsO(OH)O(OH)] layers formed from vertex- and edge-sharing [NiO(OH)] octa-hedra and [AsO(OH)] tetra-hedra as the building units. The hydrogen atom of the OH group shows occupational disorder and was refined with a site occupation factor of 1/2, indicating the equal presence of [HAsO] and [AsO] groups. Strong asymmetric hydrogen bonds between symmetry-related (O,OH) groups of the arsenate units [O⋯O = 2.588 (18) Å] as well as hydrogen bonds accepted by these (O,OH) groups from OH groups bonded to the Ni atoms [O⋯O = 2.848 (12) Å] link adjacent layers. Additional consolidation of the packing is achieved through N-H⋯O hydrogen bonds from the ammonium ion, which is sandwiched between adjacent layers [N⋯O = 2.930 (7) Å] although the H atoms could not be located in the present study. The presence of the pseudo-hexa-gonal [NiAsO(OH)O(OH)] layers may be the reason for the systematic threefold twinning of (NH)[Ni(HAsO)(AsO)(OH)] crystals. Significant overlaps of the reflections of the respective twin domains complicated the structure solution and refinement.