DISPLACEMENT OF CHLORIDE UNDER THE TRANS-EFFECT OF STRONG SIGMA-DONOR GROUPS IN PT(II) COMPLEXES

The kinetics of the reactions:. trans-[PtL2(R)Cl] + Y- → trans[PtL2(R)Y] + Cl- (R = H, C2H5, m-CF3C6H4F5; L = P(C2H5)3; Y- = Br-, I-, N-3, NO-2, CN-, SCN-, thiourea) in CH3OH at 25 °C are described. The pseudo-first-order rate constants fit the usual two-term rate law, kobs = k1 + k2[ Y-], with the k2 term strongly depending on the entering group Y-. To the extent that the term k1 can be taken as a measure of trans effect, this is found to parallel the trans-influence order of R, H > C2H5 > m-CF3C6H4 > C6F5-. This suggests a mechanism wherein the weakening of the PtCI bond trans to strong σ donors R in the ground state is the driving force of the bimolecular reaction with a weak nucleophile which is devoid of π acceptor ability, such as methanol (ground state σ-trans-effect). When Y- has good π acceptor abilities, contribution from the k2 term becomes predominant, indicating that the driving force of the reaction is now the stabilization of the 5-coordinate transition state (transition state π - trans-effect). © 1979.