The reaction of [Pt(en)(H(2)O)(2)](2+) with 1 equivalent of 1-methylcytosine at an initial pH of 6 ultimately led to the 1-methylcytosinato-bridged (ethylenediamine)platinum(II) dimer, [(en)Pt(MetCyt-H)(2)Pt(en)](NO(3))(2) (1). The species during the reactions prior to formation of 1 have been investigated by using (195)Pt NMR spectroscopy. From the reaction mixture the [Pt(en)(MetCyt)(2)](NO(3))(2) complex (3) could be isolated and structurally characterized. A rational synthesis of 1 was also achieved by dimerization of [Pt(en)(MetCyt)(H(2)O)](NO(3))(2). According to X-ray structure analysis, the Pt-Pt distance of 2.9816(3) A in 1 is well comparable with the corresponding value found in the cis-diammineplatinum(II) analog (2.981(2) A). Oxidation of 1 with K(2)S(2)O(8) led to the symmetrically sulfate-capped diplatinum(III) complex with two ethylenediamine ligands equatorially bound to the two Pt atoms ht-[(SO(4))(en)Pt(MeCyt-H)(2)Pt(en)(SO(4))] (5), as established by X-ray crystallography. When the oxidation was carried out in the presence of HClO(4), the formation of the hydroxido/aqua ht-1-methylcytosinato bridged (en)Pt(III) dimer ht-[H(2)O(en)Pt(MetCyt-H)(2)Pt(en)OH](ClO(4))(3) (6) was observed, as supported by elemental and X-ray structure analysis. In the structure of 6, two ethylenediamine ligands are differently bound to the two Pt centers: one of them equatorially while the second one caps the opposite axial site of the diplatinum(III) core. The Pt-Pt distances in the platinum(iii) dimers are 2.5982(4) in 5 and 2.5569(3) A in 6.