Complexes of the type [Pt(L)X] where L is a tridentate N ligand and X = Cl or I, were synthesized and characterized. Three of the ligands are N-derivatives of diethylenetriamine, (2-aminoethyl)(N-dimethyl-2-aminoethyl)amine, (2-aminoethyl)(N-diethyl-2-aminoethyl)amine, and (2-aminoethyl)(N-methyl-2-aminoethyl)N-methylamine. The other two ligands are di(3-amino-propyl)amine and (2-aminoethyl)(3-aminopropyl)amine. A new method for the synthesis of the chloro complexes from the direct reaction of the amine with K2[PtCl4] was developed. The reactions of these five compounds with several purine and pyrimidine bases were studied by NMR techniques. The Pt(II) complexes containing two five-membered chelates were shown to be more reactive than the one containing two six-membered rings, while the complex containing one five- and one six-membered chelates showed intermediate reactivity. For the diethylenetriamine derivatives, the complexes containing ligands with two alkyl groups on the same terminal N atom were more reactive than the one containing one methyl group on a terminal N atom and one methyl group on a non-terminal N atom.