Mechanism of the substitution of chloride in trans-[Pt(PEt3)2(R)Cl] by pyridine: kinetic evidence for a reaction intermediate

The rate of displacement of Cl– in trans-[Pt(PEt3)2(R)Cl](R = phenyl, o-tolyl, and mesityl) by pyridine has been studied in the presence of LiCl (0·005M) in methanol at 30 °C. The pseudo-first-order rate constants for approach to equilibrium (kobs) conform to the equation (i) where k1 and k4 represent the solvolytic rates of generation of an kobs=k1[py]+k4k2/k3[Cl–]//k2/k3[Cl–]+[py] intermediate from trans-[Pt(PEt3)2(R)Cl] and trans-[Pt(PEt3)2(R)(py)]+ respectively, and k2/k3 represents the relative efficiency of Cl– compared with pyridine in competing for the reactive intermediate, trans-[Pt(PEt3)2(R)-(MeOH)]+. k3 Has been measured directly from the kinetics of the reaction of trans-[Pt(PEt3)2(R)NO3] with pyridine, so that it is possible to calculate all the rate constants which appear in the expression for kobs. The consistency of the rate and equilibrium data has been checked by comparing the values of the equilibrium constants calculated from kinetic parameters with those obtained by means of direct spectrophotometric and conductance methods. The effect of increasing steric hindrance in the substrate upon the reactivity of simple nucleophiles is discussed.