Solvent effect on the rates of uncatalyzed isomerization and ligand substitution at a square planar platinum(II) complex

Abstract

In a series of papers [1] we have suggested a dissociative asynchronous mechanism for the uncatalyzed cis-trans isomerization of the complexes cis-[Pt(PEt 3) 2(R)X] (X = halide ions; R = alkyl or aryl groups). This mechanism contrasts with the well-known capability of the metal to undergo bimolecular attack and with the general mechanistic picture established for the nucleophilic displacement of ligands in square planar complexes. In this study we use solvent effect as a further criterion to distinguish between associative and dissociative reaction paths. The rates of isomerization, solvolysis and nucleophilic substitution of the complex cis-[Pt(PEt 3) 2( m-MeC 6H 4)Cl] have been measured in a series of alcohols and in acetonitrile. These processes represent three different ways for removing the chloride ion from the coordination sphere of the metal; the magnitude of the solvent effect is found to depen on the degree of charge separation occuring in the transition state. For all the solvents employed, the rates of solvolysis (path2) are of many order magnitude greater than the rates of isomerization. This large difference of reactivity, already observed and discussed in methanol [1c], argues strongly against a bimolecuar solvolysis being the initial step of the isomerization and rules out the possibility of a common intermediate for either solvolysis or isomerization. The ▪ rates of solvolysis as well as that of nucleophilic displacement by thiourea are little influenced by solvent changes, in keeping with the nature essentially associative of the activation processes, which do not require large changes of polarity or charge formation. By way of contrast, the spontaneous cis-trans isomerization of [Pt(PEt 3) 2( m-MeC 6H 4)Cl] is extremely sensitive to changes in the nature of the solvents employed, as shown by a decrease in rate of four orders or magnitude on going from methanol to 2-methyl-2-propanol. Fairly good straight lines correlate these rates and some empirical parameters which measure the relative electrophilic character of the solvents, such as α values of the Taft HBD scale or the Dimroth-Reichard's E T values. The role of the solvent is to promote the breakin of the PtCl bond in the rate determining step in leading to a cationic intermediate. Electrophilic solvation of the leaving chloride ion through nitrogen bonding is the major factor in determining the large differences in rates observed, the contribution of non specific solvent-solute interactions being negligible or constant.