Hexa­aqua­nickel(II) bis­[tri­aqua-μ(3)-oxalato-di-μ-oxalato-bariumchromate(III)] tetra­hydrate

The title compound, [Ni(H(2)O)(6)][BaCr(C(2)O(4))(3)(H(2)O)(3)](2)·4H(2)O, was obtained in the form of single crystals from the slow evaporation of an aqueous mixture of {Ba(6)(H(2)O)(17)[Cr(C(2)O(4))(3)](4)}·7H(2)O and NiSO(4)·6H(2)O in the molar ratio 1:4. Its structure is made up of corrugated anionic (101) layers of formula [BaCr(C(2)O(4))(3)(H(2)O)(3)](n)(n) (−) that leave voids accommodating the charge-compensating cations, [Ni(H(2)O)(6)](2+) (point group symmetry [Image: see text]), as well as the water mol­ecules of crystallization. The anionic layers are built from the connection of barium and chromium atoms through bridging oxalate ligands. The Cr(III) atom is hexa­coordinated by O atoms of three oxalate ligands while the Ba(II) atom is tenfold coordinated by three O atoms of water mol­ecules and seven O atoms of four oxalate ligands. Each Ni(II) atom sits on an inversion center and is coordinated by six water mol­ecules. One of the uncoordinated water mol­ecules is disordered over two sites, with a refined occupancy ratio of 0.51 (5):0.49 (5). In the crystal, extensive O—H⋯O hydrogen-bonding inter­actions link the anionic layers, the charge-balancing cations as well as the water mol­ecules of crystallization into a three-dimensional supra­molecular network.