2,2'-{1,1'-[2,2'-Oxalylbis(hydrazin-2-yl-1-yl-idene)]diethyl-idyne}dipyridinium bis-(perchlorate) dihydrate.

The title salt, C(16)H(18)N(6)O(2) (2+)·2ClO(4) (-)·2H(2)O, was obtained unintentionally as a major product in the reaction of Zn(ClO(4))(2)·6H(2)O with the N',N'(2)-bis-[(1E)-1-(2-pyrid-yl)ethyl-idene]ethanedihydrazide (H(2)L) ligand. The (H(4)L)(2+) cation lies across a centre of inversion. The pyridiniumimine fragments of (H(4)L)(2+) adopt syn orientations. Intra-molecular N-H⋯N and N-H⋯O hydrogen bonds lead to the formation of S(5) motifs. In the crystal, neighbouring cations are connected by π-π inter-actions between pyridinium units with a centroid-centroid distance of 3.600 (1) Å. Moreover, the crystal components are assembled into two-dimensional layers via N-H⋯O and O-H⋯O hydrogen bonds, with no direct hydrogen-bonding inter-actions between cations.