tran-[a(2)PtL(LH)]NO3.xH(2)O (a = NH3, L = 2-pyridonate, C5H4NO, LH = 2-hydroxypyridine, C5H5NO, x = 2 (1); a = CH3NH2, x = 0 (2)) have been prepared, and Cu-II adducts of composition trans-[a(2)PtL(2)CuL(2)Pta(2)] (NO3)(2) (a = NH3, (3); a CH3NH2 (4)) have been isolated. X-ray structures of 2, 3, and 4 are reported. Crystal data are as follows : 2, orthorombic system, space group Pccn, a = 8.486(1) Angstrom, b = 13.200(3) Angstrom, c = 13.920(1) Angstrom, Z = 4 : 3, orthorombic, space group Fdd2, a = 23.281(4) Angstrom, b = 22.551(4) Angstrom, c = 10.852(2) Angstrom, Z = 8; 4, monoclinic system, space group P2(1)/a, a = 11.448(4) Angstrom, b = 18.771(2) Angstrom, c = 16.085(3) Angstrom, beta = 105.81(2), Z = 4. The cation of 2, in which the two heterocyclic ligands bind to Pt via N(1) positions and adopt a coplanar head-tail arrangement, is structurally not unusual. In the crystal, cations of 2 are associated by very short intermolecular H bonds of 2.443(9) Angstrom into infinite chains. Since the acidic proton of this H bond was not located, a differentiation between a symmetric single minimum H bond and a double minimum situation (with H disordered) Was not possible, but IR spectroscopy (no deuterium shift of nu(OH) at ca. 750 cm(-1)) favors the latter. The Cu-II ions in 3 and 4 are coordinated via 0(2) oxygens of the four 2-pyridonato ligands. Geometries and several spectroscopic properties of 3 and 4 differ markedly from the corresponding complexes of composition cis-[a(2)-Pt(L’)(2)Cu(L’)(2)Pta(2)](2+) (L’ = bridging pyrimidine nucleobase; coordination of Pt via endocyclic ring N, of Cu via exocyclic O) in the following ways : Coordination spheres of both Pt-II and Cu-II, which are essentially square planar in the cis-compounds, have undergone considerable distortions in 3, and 4. Pt-Cu distances in 3 (2.6312-(4) Angstrom) and 4 (2.6320(8) and 2.6450(8) Angstrom) are significantly shorter. The trans-compounds 3 and 4 are of orange-red colors and exhibit d-d transitions at considerably lower energy than the cis-compounds which are greenish. EPR spectra (g values, A values) are also different and the powder spectrum of 3 is unusual, being a superposition of signals due to the molecular g-tensor, and weak peaks due to an exchange-narrowed g-tensor.