To what extent can the Pt–C bond of a metallacycle labilize the trans position? A temperature- and pressure-dependent mechanistic study

The orthoplatinated complex [Pt{C6H3X(CH2NMe2)}(NC5H4SO3-3)(H2O)](X = H 1a or 3-MeO 1b) was designed for mechanistic studies in water. The aqua ligand is located trans to the Pt–C bond of the phenyl group which lies in the platinum(II) co-ordination plane. The rates of substitution of the aqua ligand by a series of nucleophiles (Nu)(Cl–, Br–, I–, N3–, SCN–, thiourea, N,N′-dimethylthiourea or N,N,N′,N′-tetramethylthiourea) were studied as a function of concentration, pH, temperature, and pressure by using a stopped-flow technique. The pKa value of the aqua ligand in 1a is 9.75 ± 0.05 and the observed pseudo-first-order rate constants for the substitution reaction are given by kobs=k1[Nu]+k–1. The k–1 term arises from the reverse solvoiysis reaction and is insignificant for stronger, S-donor nucleophiles. The values of k1 are ca. four orders of magnitude higher than the corresponding rate constants for anation reactions of [Pt(dien)(H2O)]2+(dien = diethylenetriamine) and close to the rate constants for anation of [Pd(dien)(H2O)]2+. The effect is largely due to a strong decrease in ΔH‡. An analysis of the activation parameters, ΔH‡, ΔS‡ and ΔV‡, clearly shows that the substantial rate increase is not associated with a changeover in mechanism and that the substitution process is still associative in nature.