Reaction mechanism for olefin exchange at chloro ethene complexes of platinum(II)

Complex equilibria in methanol/chloroform/dichloromethane solutions containing Zeise's anion, [PtCl3(C2H4)](-)(1), the solvento species, trans-[PtCl2(C2H4)(MeOH)] (2), and the dinuclear complex, trans-[PtCl2(C2H4)](2) (3), have been studied by UV-vis, H-1, and Pt-195 NMR spectroscopy, giving average values of K-Cl = (1.6 +/- 0.2)10(3) M-1 and K-S = (0.16 +/- 0.02) M-1 for the equilibrium constants between 2 and 1 and 3 and 2, respectively. The bridged complex 3 is completely split into monomeric solvento complexes 2 in methanol and in chloroform or dichloromethane solutions with [MeOH] > 0.5 M. Ethene exchange at the mononuclear complexes 1 and 2 was studied by H-1 NMR line-broadening experiments in methanol-d(4). Observed overall exchange rate constants decrease with an increase in free chloride concentration due to the displacement of the rapid equilibrium between 1 and 2 toward the more slowly exchanging parent chloro complex 1. Ethene exchange rate constants at 298 K for complexes 1 and 2 are k(ex1) = (2.1 +/- 0.1)10(3) M-1 s(-1) and k(ex2) = (5.0 +/- 0.2)10(5) M-1 s(-1), respectively, with corresponding activation parameters Delta H(1)double dagger = 19.1 +/- 0.3 kJ mol(-1), Delta S(1)double dagger = -117 +/- 1 J K-1 mol(-1), Delta H(2)double dagger = 10.2 +/- 0.4 kJ mol(-1), and Delta S(2)double dagger = -102 +/- 2 J K-1 mol(-1). The activation process is largely entropy controlled; the enthalpy contributions only amounting to approximate to 30% of the free energy of activation. Ethene exchange takes place via associative attack by the entering olefin at the labile site trans to the coordinated ethene, which is either occupied by a chloride or a methanol molecule in the ground state. The intimate mechanism might involve a two-step process via trans-[PtCl2(C2H4)(2)] in steady state or a concerted process via a pentacoordinated transition state with two ethene molecules bound to the platinum(II).