A series of salts of the type [Pt(terpy)Me]X (X = Cl, NO3, PF6, ClO4, B(C6H5)(4)) containing a new organometallic complex cation of platinum(II) with 2,2’:6’,2 "-terpyridine (terpy) were synthesized and characterized by H-1, C-13, and Pt-195 NMR spectroscopy. No evidence for fluxionality of the terpyridine ligand was obtained, indicating that it is terdentate. The fourth position in the coordination plane is occupied by a methyl group. The resulting cation is unreactive toward substitution and is stable in aqueous solutions under mild conditions. The planarity and electron delocalization of the terpy moiety lead to extensive stacking interactions, forming dimers in dilute aqueous solution and larger aggregates when the concentration and/or the ionic strength increase. UV/vis and H-1 NMR spectra show characteristic dependencies on the concentration of the complex, the temperature, the solvent, and the ionic strength. Analysis of the absorption spectral data gives a value of 10 (+/-8) x 10(3) M(-1) (T = 298 K; mu = 0.101 mol L(-1)) for the dimerization equilibrium constant. Resonance light-scattering spectra, measured for the first time on a metal-containing noncyclic substrate, provide evidence for the tendency of the complex to form large self-aggregates even under low ionic strength conditions. The interaction of the cationic complex ion with calf thymus DNA was investigated by UV/vis, CD spectroscopy, resonance light-scattering, thermal denaturation, and gel electrophoresis mobility assays. At high r(f) ratios the complex seems to form extended aggregates on the surface of the nucleic acid, but at lower r(f) ratios evidence was obtained for intercalation.