The Mechanism of Thermal Decomposition of tra ns - Chloroethylbis ( triethylphosphine ) platinum ( I I )

The thermal decomposition of trans-chloroethylbis(triethylphosphine)platinum(II) (L2PtEtCl, 1) in cyclohexane solution at 158 oC yields trons-chlorohydridobis(triethylphosphine)platinum(II) (3) and ethylene. The most probable rate-limiting step for this reaction is dissociation of ethylene from a five-coordinate l8-electron plat inum(Il) intermediate, L2PtHCl(CzH) Q). Three pathways might be involved in the formation of 2 from I (Scheme I): p-hydride elimination proceeding directly from I without prior dissociation of phosphine or chloride; phosphine dissociat ion, reversible p-hydride el imination, and phosphine reassociation; or chloride dissociation, reversible B-hydride elimination, and chloride reassociation. Presently available evidence does not distinguish between these three pathways. The conversion of I to 3 and ethylene has Arrhenius activation parameters E" = 34 kcaU mol and log A = 13. The reaction is effectively irreversible at low to moderate conversions, when ethylene concentrations in solution are low. At high conversions, the reverse reaction (3 + ethylene + l) is significant. The equilibrium constant for I :3 + ethylene was measured as a function of temperature giving K"o(158 oC) = 0.69 M, lG"(158 "C) = 0.32 kcal/mol, AHo = L2kcillmol, and ASo = 27 eu. The deuterium equilibrium isotope eft'ect for the reaction L2PtClC2Hi(Ds) LzPtClH(D) + C2H4(D*) is KuoHTKuoo = 1.9 + 0.4; the deuterium hinetic isotope effect on the overal l rate of decomposition of LrPtClCrHs(Ds) under conditions in which the reverse reaction is slow is kslhs = 2.5 + 0.2. The rate of reaction increases slightly with increasing concentration of phosphine and chloride ion (as benzyldimethyltetradecylammonium chloride) and with solvent polarity. Intermolecular exchange of phosphine ligands between platinum centers occurs within 30 min at room temperature in cyclohexane solution (measured by observing the NMR spectrum of mixtures of L2PtEtCl and (L-d1b)2PtEtCl); halide ion exchange between L2Pt(C2Hs)Cl and L2PI(C2DJBr is also observed within 30 min under these conditions.