Arsenates of Divalent Metals Comprising Arsenic Acid—An Update

Divalent metal oxidoarsenates(V) with compositions M(H₂AsO₄)₂(H₃AsO₄)₂ (M = Mg, Mn, Co, Ni), M(HAsO₄)(H₃AsO₄)(H₂O)_0.5 (M = Mn, Cd) and Zn(HAsO₄)(H₃AsO₄) were obtained from solutions containing an excess of arsenic acid. Single crystal X-ray diffraction revealed isotypism of the M(H₂AsO₄)₂(H₃AsO₄)₂ (M = Mg, Mn, Co, Ni) structures with the known Cu and Zn members of this series whereas M(HAsO₄)(H₃AsO₄)(H₂O)_0.5 (M = Mn, Cd) and Zn(HAsO₄)(H₃AsO₄) crystallize in novel structure types. The two isotypic M(HAsO₄)(H₃AsO₄)(H₂O)_0.5 (M = Mn, Cd) structures are closely related with that of Zn(HAsO₄)(H₃AsO₄). Both comprise undulating centrosymmetric $[_{\propto }^1$ MO_4/2O_2/1] chains that share corners with HAsO₄²⁻ tetrahedra and H₃AsO₄ tetrahedra to build up layers extending along (001). Intermediate water molecules (occupancy 0.5) link adjacent layers in the water-containing compound whereas the linkage in the Zn-compound is mediated by weak hydrogen bonding interactions between the layers. Results of a quantitative comparison between all known structures of the M(H₂XO₄)₂(H₃XO₄)₂ (M = Mg, Mn, Co, Ni, Cu, Zn; X = P, As) series as well as between the two M(HAsO₄)(H₃AsO₄)(H₂O)_0.5 (M = Mn, Cd) structures are presented.