A mixed-purine nucleobase complex of composition trans-[(NH3)(2)-Pt(9-EtA-N1)(9-MeHx-N7)](NO3)(2).2H(2)O (1) (9-EtA = 9-ethyladenine; 9-MeHx = 9-methylhypoxanthine) has been prepared and characterized by X-ray crystallography. Cations of 1 are self-complementary as far as hydrogen bonding properties are concerned and form H bonded dimers, containing four intermolecular hydrogen bonds in addition to two intramolecular ones. The resulting mixed-purine square is considered a model compound for a putative mixed-purine tetrad consisting of two adenines and two guanines. In this model, the one-metal, four-nucleobase quartet motif, as seen in guanine or uracil quartets of nucleic acids, with the metal located in the center of the base tetrad, has been altered to a two-metal, four-nucleobase motif, with the two metal ions localized at the periphery.