Diaquabis(pyridine-2-carboxamide-kappa2N1,O2)nickel(II) disaccharinate tetrahydrate

In the crystal structure of the title compound, [Ni(C6H6N2O)2(H2O)2](C7H4NO3S)2.4H2O or [Ni(pia)2(H2O)2](sac)2.4H2O (pia is picolinamide or pyridine-2-carboxamide, and sac is the saccharinate anion), the Ni2+ cation, located on a centre of symmetry, is coordinated by two symmetry-related aqua ligands together with a pair of symmetry-related bidentate pia molecules and exhibits a distorted octahedral environment. The unique unligated sac anion in the asymmetric unit resides on a general position and has a single negative charge. The coordinated water molecules link the sac ions to the metal complex via O-H...O hydrogen bonds. In addition, the sac ions are linked to the metal complex via intermolecular pi-pi interactions between the benzene ring of the sac ion and the pyridine ring of a pia ligand. Each uncoordinated water molecule is hydrogen bonded to sac moieties through O-H...O and O-H...N hydrogen bonds.