A heteronuclear complex of composition trans-[(NH3)(2)Pt(N4-1-MeC--N3)(2)Cu(H2O)(2)](ClO4)2 (1a), with 1-MeC- = 1-methylcytosinate, has been prepared and characterized by X-ray crystallography. 1a (Cu,Pt) is a linkage isomer of a previously described compound with the two metals inverted (Pt,Cu). The intermetallic distances are significantly different in the two types of compounds, 2.6109(9) Angstrom in 1a, yet 2.49-2.56 Angstrom in several forms of the linkage isomer. When heated in water in the presence of air, la is converted in low yield into diplatinum(III) compounds [(H3N)Pt(1-MeC--N3,N4)(4)Pt(NH3)](2+) (2a) and [(H2O)Pt(1-MeC--N3,N4)(4)Pt(NH3)](2+) (2b), which were crystallized as ClO4- salts. In a modified procedure a third representative of this g,roup of diplatinum(III) compounds, [(O2N)Pt(1-MeC--N3,N4)(4)Pt](+) (2c) was isolated. All three compounds contain the four bridging 1-MeC ligands in a head, tail, head, tail arrangement with Pt-Pt distances (2.4516(7)-2.4976(9) Angstrom) that are the shortest ones among diplatinum(III) compounds containing nucleobases.