The X-ray crystal structures of two complexes of the composition trans-[Ft(NH3)(2)(9-EtG-N7)(1-MeC-N3＇)]X . nH(2)O (1) with 9-EtG = 9-ethylguaninate and 1-MeC = 1-methylcytosine are reported. 1b (X picrate, n = 1) crystallizes to produce a dimetalated base quartet, held together by H-bonding interactions between pairs of cations. This feature essentially corresponds to the solution structure previously proposed by us on the basis of H-1 NMR and ESI-MS, with a H-bonding interaction between the aromatic H5＇ proton of 1-MeC and the deprotonated N1 position of 9-EtG. 1c (X = trifluoromethanesulfonate, n = 0) crystallizes in a radically different fashion as a consequence of nucleobase rotation about the Pt-N bond, leading to a reversed Hoogsteen arrangement without any intracomplex H bonding between the two bases. In the solid-state structure of 1b short intermolecular H bonds exist between the exocyclic NH2 group of 1-MeC and 06 of 9-EtG (2.715(7) Angstrom). Considerably lon er intra- (N4＇(1-MeC)... 06(9-EtG), 3.229(6) Angstrom) and intermolecular (C5＇(1-MeC)... N1(9-EtC), 3.548(7) Angstrom) H bonds are primarily a consequence of considerable base twisting, presumably caused by stacking between the guanine residues and the picrate anions. In DMSO-d(6) solution, the cyclic base quartet structure is favored, regardless of the nature of the anion X (X = picrate, trifluoromethanesulfonate, perchlorate, nitrate). An association constant K-D = 44.1 +/- 3.2 M-1 for the dimerization has been determined.