Kinetics of Carbon Monoxide Exchange in Chloro and Bromo Carbonyl Complexes of Palladium(II) and Platinum(II)

Carbonyl exchange between [MX(3)((CO)-C-13)](-) (MX = PdCl, PtCl or PtBr) and (CO)-C-12 has been studied by IR and C-13 NMR spectroscopies as a function of temperature. The exchange is first-order in both CO and complex concentration, and the activation entropies are negative, suggesting an associative reaction mechanism. There is a small solvent effect. but no chloride dependence. Activation parameters and rate constants, in the order Delta H-double dagger/kJ mol(-1), Delta S-double dagger/J K-1 mol(-1), k(ex)(298)/dm(3) mol(-1) s(-1), are for [PdCl3(CO)](-) 60 +/- 8, -47 +/- 30, 0.57 +/- 0.07; for [PtCl3(CO)](-) 60 +/- 7. -91 +/- 22, (3.3 +/- 0.3) x 10(-3) and for [PtBr3(CO)](-) 49 +/- 6, -104 +/- 19, (6.0 + 0.2) x 10(-2) (95% confidence interval). This is three to six orders of magnitude slower than ethene exchange in analogous ethene halide complexes.