The reaction of nickel(II) nitrate hexahydrate with the protontransfer compound piperazinediium pyridine-2,6-dicarboxylate, or (pipzH2)(pydc) (in which pipz is piperazine and pydcH2 is pyridine-2,6-dicarboxylic acid), in aqueous solution leads to the formation of the title compound, (pipzH2)[Ni(pydc)2]- 4H2O or (C4H12N2)[Ni(C7H3NO4)2]4H2O. The anion is a sixcoordinate complex with a distorted octahedral geometry around NiII. The torsion angles show that the two (pydc)2– units are almost perpendicular to each other. Considerable – stacking interactions between two aromatic rings of (pydc)2–, with distances of 3.4686 (14) and 3.5034 (14) A ˚ , are observed. Extensive intermolecular O—H O, N—H O and C—H O hydrogen bonding involving the (pydc)2– ligand, (pipzH2)2+ as counter-ion and uncoordinated water molecules connect the various components into a supramolecular structure
