SUBSTITUENT EFFECTS ON THE RATES OF UNCATALYZED CIS TO TRANS ISOMERIZATION AND METHANOL SOLVOLYSIS OF ARYLPLATINUM COMPLEXES - INFERENCES ON THE NATURE OF THE ARYL-METAL BOND

The complexes cis-[Pt(PEt3)2(YC6H4)X] (X = Cl, Br, I and Y = H; X = CI and Y = H, m-Me, p-Me, m-OMe, p-OMe, m-F, p-F, w-Cl, p-Cl, m-CF3, p-CF3) spontaneously convert to their trans isomers in methanol, and for all systems the isomeric equilibrium lies well over to the transform. Large values of enthalpy of activation and positive entropies of activation are associated with isomerization. The rates of isomerization (k1 s-1) suffer mass-law retardation by X and arc sensitive either to the nature of the halide ion or to changes in the electron density at the metal, brought about by para and meta substituents on the aromatic ring. The same factors hardly affect the rates of solvolysis (ki s-1) which are at least 103times higher than k1 and are characterized by low enthalpies and largely negative entropiesof activation. These findings confirm that uncatalyzed isomerization of cis-[Pt(PEt3)2(aryl)X] complexes involves as a rate-determining step the breaking of the Pt-X bond followed by the interconversion of two T-shaped three-coordinate (i.e., 14-electron) intermediates. Correlation of ki with Hammett&s parameters of the Y substituents or with the set of Swain and Lupton dual-substituent parameters indicates that a rather than π interactions are dominant in the arylplatinum bond. © 1979, American Chemical Society. All rights reserved.