The reaction of K2PtCl4 with an excess of 1-methyluracilate (1-MeU) in water at 60 degrees C leads to the formation of two major products, K-2[Pt(1-MeU-N3)(4)]center dot 10H(2)O (1) and trans-K[Pt(1-MeU-N3)(2)(1-MeU-C5)(H2O)]-3H(2)O (2). Addition of CuCl2 to an aqueous solution of 2 yields the mixed-metal complex trans-[PtCl(1-MeU-N3,O4)(2)(1-MeU-C5,O4)Cu-(H2O)]center dot H2O (4). Single-crystal X-ray analysis was carried out for 1 and 4. In both compounds, the heterometals (K+ in 1 and Cu2+ in 4) are bonded to exocyclic oxygens atoms of the 1-MeU ligands, giving rise to intermetallic distances of 3.386(2) and 3.528(2) angstrom in 1 and 2.458(1) angstrom in 4. The shortness of the Pt-Cu separation in 4 is consistent with a dative bond between Pt-II and Cu-II. The aqua ligand in 2 is readily substituted by a series of other ligands (e.g., 1-MeC, 9-MeGH, and CN-), as demonstrated by H-1 NMR spectroscopy, with (3)J(Pt-195-H-1(6)) coupling constants being sensitive indicators. Acid-base equilibria of 1 and 2 have been studied in detail and reveal some unexpected features: 1 has a relatively high basicity, with protonation starting below pH 5, and first and second pK(a) values being ca. 3.4 and 0.4, respectively. These pKa values are markedly higher than those of related neutral 2:1 or cationic 1:1 complexes and are attributed to both charge effects (-2 charge of 1) and a favorable stabilization of oxygen-protonated species by the arrangement of four exocyclic oxygen groups of 1-MeU ligands at either sides of the platinum coordination planes. Whereas in 2, H+ affinities of the three uracil ligands are in the normal range, there is a surprisingly low acidity of N(3)H of the C5-bonded uracil with a pK(a) of similar or equal to 12.2, which compares with 9.75 for free 1-methyluracil. This implies that the C5-bonded Pt-II does not induce the typical acidifying effect of a Pt-II metal entity when bonded to a ring nitrogen atom of a neutral nucleobase. Rather, the effect is qualitatively similar to that of a metal ion bonded to N3 of an anionic 1-MeU ligand, which likewise increases its overall basicity as compared to neutral 1-MeUH.