Bis(guanidinium) bis­(4-hydroxy­pyridine-2,6-dicarboxyl­ato-κ(3) O (2),N,O (6))nickel­ate(II) dihydrate

The reaction of nickel(II) nitrate hexa­hydrate, guanidine (G) and 4-hydroxy­pyridine-2,6-dicarboxylic acid (hypydcH(2)) in a 1:2:2 molar ratio in aqueous solution resulted in the formation of the title compound, (CH(6)N(3))(2)[Ni(C(7)H(3)NO(5))(2)]·2H(2)O or (GH)(2)[Ni(hypydc)(2)]·2H(2)O. The six donor atoms of the two 4-hydroxy­pyridine-2,6-dicarboxyl­ate or (hypydc)(2−) ligands form a distorted octa­hedral arrangement around the Ni(II) centre. Considerable C—O⋯π stacking inter­actions between the CO groups of carboxyl­ate fragments and the pyridine rings of (hypydc)(2−) with a distance of 3.3212 (8) Å are observed. In the crystal structure, a wide range of noncovalent inter­actions consisting of hydrogen bonding (of the types O—H⋯O and N—H⋯O), ion pairing, and π–π [centroid–centroid distance 3.8037 (5) Å], N—H⋯π and C—O⋯π stacking inter­actions connect the various components into a supra­molecular structure.