Key indicators: single-crystal X-ray study; T = 100 K; mean (C-C) = 0.002 Å; some non-H atoms missing; disorder in solvent or counterion; R factor = 0.037; wR factor = 0.100; data-to-parameter ratio = 22.6.The asymmetric unit of the title compound, {(C 2 H 10 N 2 )-[Cu(C 10 H 2 O 8 )]Á2.5H 2 O} n , contains one cuprate(II)-4benzene-1,2,4,5-tetracarboxylate anion, one ethylenediammonium cation and two and a half water molecules. The Cu atom is four-coordinated by four O atoms from four benzene-1,2,4,5-tetracarboxylate anions. Intra-and intermolecular O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds in the formation of a supramolecular structure. One of the water molecules is disordered equally over two sites.

In the cation of the title compound, [Au(C7H9N)2][AuCl4], the AuI atom is two-coordinated in a linear arrangement by two N atoms from two 2,6-dimethyl­pyridine ligands. In the anion, the AuIII atom has a virtually square-planar coordination geometry. The Au atoms both are located on centers of inversion. The crystal structure involves inter­molecular C—H⋯Cl hydrogen bonds.