The X-ray crystal structures of four mixed-nucleobase complexes 1a, 1b, 2, and 4 of trans-a(2)Pt(II) (a = NH3 or CH3NH2) with N1-bonded 7,9-dimethylguanine (7,9-DimeG) and N7-bonded 9-ethylguanine (9-EtGH) are reported. The compounds are discussed in terms of their possible model character for platinated nucleobase triplets of the pu x pu.pym (pu = purine, pym = pyrimidine) type and are compared with known guanine guanine pairs. Structural differences resulting from a "metal modification" have been studied, as have been stacking and H bonding motifs. 1a acid 1b are two modifications of the same compound, trans-[(NH3)(2)Pt(7,9-DimeG)(9-EtGH)](ClO4)(2). H2O. X-ray data for aa: orthorhombic, Pca2(1), a 13.833(3) Angstrom, b = 14.357(3) Angstrom, c = 13.293(3) Angstrom, Z = 4, R = 0.027. 1b: monoclinic, P2(1)/n, a = 13.987(3) Angstrom, b = 12.582(3) Angstrom, c = 15.613(3) Angstrom, beta = 104.61(3)degrees, Z = 4, R = 0.034. trans-[(CH3NH2)(2)Pt(7,9-DimeG)(9-EtGH)](ClO4)(2) (2): orthorhombic, Pca2(1), a = 13.886(1) Angstrom, b = 14.698(1) Angstrom, c = 13.008(3) Angstrom, Z = 4, R = 0.034. trans-[a(2)Pt(7,9-DimeG)(9-EtG)](ClO4). nH(2)O (3, a = NH3, n = 2; 4, a = CH3NH2, n = 3). X-ray data for 4: monoclinic, C2/c, a = 23.296(5) Angstrom, b = 10.566(2) Angstrom, c = 22.327(4) Angstrom, beta = 93.73(3)degrees, Z = 8, R = 0.047. The metalated base triplet 2-(1-MeC) (1-MeC = 1-methylcytosine) has been detected in DMSO-d(6) solution through H-1 NMR spectroscopy, and an ab initio optimized structure of an analogous platinated base triplet has been obtained.