The hydroxo complex cis-[L2Pt(mu-OH)](2)(NO3)(2)(L = PMe2Ph), in various solvents, reacts with 1-methylcytosine (1-MeCy) to give as the final product the cyclic species cis-[L2Pt{1-MeCy(-H), (NN4)-N-3}](3)(NO3)(3) (1) in high or quantitative yield. X-ray analysis of 1 evidences a trinuclear species with the NH2-deprotonated nucleobases bridging symmetrically the metal centers through the N3 and N4 donors. A multinuclear NMR study of the reaction in DMSO-d(6) reveals the initial formation of the dinuclear species cis-[L2Pt{1-MeCy(-H), (NN4)-N-3}](2)(2+) (2), which quantitatively converts into 1 following a first-order kinetic law (at 50 degrees C, t(1/2) = 5 h). In chlorinated solvents, the deprotonation of the nucleobase affords as the major product (60-70%) the linkage isomer of 1, cis-[L2Pt{1-MeCy(-H)}](3)(3+) (3), in which three cytosinate ligands bridge unsymmetrically three cis-L2Pt2+ units. In solution, 3 slowly converts quantitatively into the thermodynamically more stable isomer 1. No polynuclear adducts were obtained with the hydroxo complex stabilized by PPh3. cis-[(PPh3)(2)Pt(mu-OH)](2)(NO3)(2) reacts with 1-MeCy, in DMSO or CH2Cl2, to give the mononuclear species cis-[(PPh3)(2)Pt{1-MeCy(-H)}(1-MeCy)](NO3) (4) containing one neutral and one NH2-deprotonated 1-MeCy molecule, coordinated to the same metal center at the N3 and N4 sites, respectively. X-ray analysis and NMR studies show an intramolecular H bond between the N4 amino group and the uncoordinated N3 atom of the two nucleobases.