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The correspondence author in the paper by Jia et al. [Acta Cryst. (2010), E66, o3315] is corrected.

The molecule of the title compound, C4H2N2O4, is located around an inversion center and the four O atoms are in the 2,3,5,6-positions of the piperazine ring. In the crystal, bifurcated N—H...O hydrogen bonds link the molecules into a corrugated layer parallel to (101).

In the molecular structure of the title compound, [CuCl(C6H5NO)4]Cl, the CuII atom is coordinated by four N atoms of four pyridine-4-carboxaldehyde ligands and one chloride anion in a slightly distorted square-pyramidal coordination geometry. There is also a non-coordinating Cl− anion in the crystal structure. The CuII atom and both Cl atoms are situated on fourfold rotation axes. A weak C—H...Cl interaction is also present.

The title complex, [Co(C12H9N2O3S)2]·2H2O, has site symmetry 2 with the CoII cation located on a twofold rotation axis. Two tridentate 2-(2-pyridylmethyleneamino)benzenesulfonate (paba) ligands chelate to the CoII cation in a distorted octahedral geometry. The pyridine and benzene rings in the paba ligand are oriented at a dihedral angle of 42.86 (13)°. Intermolecular O—H...O and C—H...O hydrogen bonding is present in the crystal structure.

In the mononuclear title compound, [Cu(C12H9N2O3S)2], the copper(II) salt of 2-(2-pyridylmethyleneamino)benzenesulfonic acid, the CuII atom is coordinated by one O and two N atoms from a monoanion as well as by two N atoms from another monoanion in a distorted trigonal-bipyramidal environment.