Synthesis and crystal structure of (NH 4 )[Ni 3 (HAsO 4 )(AsO 4 )(OH) 2 ].

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 ₃ (HAsO ₄ )(AsO ₄ )(OH) ₂ ] and is characterized by pseudo-hexa-gonal (001) ² _∞ [Ni ₃ As ₂ O _18/3 (OH) _6/3 O _1/1 (OH) _1/1 ] ^- layers formed from vertex- and edge-sharing [NiO ₄ (OH) ₂ ] octa-hedra and [AsO _3.5 (OH) _0.5 ] 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 ₄ ] ^2- and [AsO ₄ ] ^3- 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 ^II 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 ² _∞ [Ni ₃ As ₂ O _18/3 (OH) _6/3 O _1/1 (OH) _1/1 ] ^- layers may be the reason for the systematic threefold twinning of (NH ₄ )[Ni ₃ (HAsO ₄ )(AsO ₄ )(OH) ₂ ] crystals. Significant overlaps of the reflections of the respective twin domains complicated the structure solution and refinement.
(© Eder and Weil 2024.)