Reaction Mechanism for Olefin Exchange at Chloro Ethene Complexes of Platinum(II)

Synopsis  In methanol solutions of Zeise's salt, the parent complex [PtCl3(C2H4)]- as well as its solvolysis product, trans-[PtCl2(C2H4)(MeOH)], exchange rapidly with free ethene. The olefin exchange process, which is largely entropy controlled, involves the substitution labile site trans to the coordinated ethene.    Abstract   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, 1H, and 195Pt NMR spectroscopy, giving average values of KCl = (1.6 ± 0.2)103 M-1 and KS = (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 1H NMR line-broadening experiments in methanol-d4. 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 kex1 = (2.1 ± 0.1)103 M-1 s-1and kex2 = (5.0 ± 0.2)105 M-1 s-1, respectively, with corresponding activation parameters ΔH1⧧ = 19.1 ± 0.3 kJ mol-1, ΔS1⧧ = −117 ± 1 J K-1 mol-1, ΔH2⧧ = 10.2 ± 0.4 kJ mol-1, and ΔS2⧧ = −102 ± 2 J K-1 mol-1. The activation process is largely entropy controlled; the enthalpy contributions only amounting 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)