Cationic ethylene platinum(II) complexes of the type [Pt(CNN)(C2H4)](+), containing a methyl fragment and different diimines (NN), or terdentate (kappa C-kappa(NN)-N-2') anionic ligands, were synthesized and fully characterized both as solids and in solution [NN = 2,2'-dipyddylamine, 1; 2,2'-dipyridylsulfide, 2; 1,10-phenanthroline, 3; 4,7-diphenyl-1,10-phenanthroline, 4; 3,4,7,8-tetramethyl-1,10-phenanthroline, 5; 2,2'-bipyridine, 6; HC-NN = 6-tert-butyl-2,2'-bipyridine, 7; 6-neo-pentyl-2,2'-bipyridine, 8; 6-phenyl-2,2'-bipyridine, 9; 6-(alpha-methyl)benzyl-2,2'-bipyridine, 10; 6-((x-ethyl)benzyl-2,2'-bipy dine, 11; 6-(alpha,alpha-dimethyl)benzyl-2,2'-bipyhdine, 12]. Crystals suitable for X-ray analysis of complexes 5 and 7 were obtained. Ethene exchange at the cyclometalated platinum(II) complexes 7, 8, and 10-12 was studied by H-1 NMR line-broadening experiments in chloroform-d, as a function of both temperature and olefin concentrations. For the other prepared complexes the process was too fast to be monitored on the NM R time scale even at the lowest temperature. The ethylene exchange rates show a linear dependence on the concentration of the free ligand, with a negligible k(1) term indicating that either a solvolytic or a dissociative pathway to the products is absent or negligible. The values of the second-order rate constants kexc, as obtained by linear regression analysis of the experimental data at 298 K, are in a range of ca. 10(4) - 10(5) s(-1) m(-1). The activation entropies are negative, ranging between - 129 and - 112 J K-1 mol(-1), as expected for associative processes. The activation process is largely entropy controlled: the TAS* contribution to the free energy of activation is extremely large, amounting to more than 80% for all complexes, with a smaller enthalpy contribution. All the experimental findings evidence that the mechanism takes place via an associative attack by the entering olefin, through a well-ordered, stable pentacoordinated transition state with the two ethene molecules on the trigonal plane. The reactivity of [Pt(CNN)(C2H4)](+) complexes is strongly dependent on the choice of coordinated 6-substituted-2,2'-bipyridines, especially when the terdentate anionic fragment is capable of generating steric crowding and congestion on the coordination plane.