A Kinetic Study of the Competition between Carbon Monoxide and Alkenes in Chloro Complexes of Platinum(II)

Alkenes (cyclohexene; 1-octene) react with cis-PtCl2(CO)(2), leading to cis-PtCl2(CO)(alkene) (1a, alkene) cyclohexene; 1b, alkene = 1-octene). The substitution kinetics to 1a and 1b in 1,2-dichloroethane in the temperature range 273-308 K have the following second-order rate constants at 298 K, respectively: cyclohexene, (0.94 +/- 0.07) x 10(-3) M-1 s(-1); 1-octene, (2.33 +/- 0.05) x 10(-2) M-1 s(-1). The activation parameters are Delta H* = 37 +/- 4 kJ mol(-1), Delta S* = -180 +/- 10 J K-1 mol(-1) for the reaction affording 1a and Delta H* = 36.5 +/- 0.7 kJ mol(-1), Delta S* = -154 +/- 2 J K-1 mol(-1) for the reaction to 1b. The large negative values of the activation entropies are consistent with an associative mechanism. The reverse reactions proceed with a similar rate constant.