Bis(guanidinium) bis­(4-hydroxy­pyridine-2,6-dicarboxyl­ato-κ3 O 2,N,O 6)nickel­ate(II) dihydrate

The reaction of nickel(II) nitrate hexa-hydrate, guanidine (G) and 4-hydroxy-pyridine-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-hydroxy-pyridine-2,6-dicarboxyl-ate or (hypydc)(2-) ligands form a distorted octa-hedral arrangement around the Ni(II) centre. Considerable C-O⋯π stacking inter-actions between the CO groups of carboxyl-ate 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 inter-actions 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 inter-actions connect the various components into a supra-molecular structure.