Substitution reactions on cyclometallated Pt(IV) complexes; a kinetic survey of the bond strength of PtS, N, and P-based ligands

The substitution reactions of a series of sulfur, nitrogen and phosphorus-based ligands (L) on Pt(IV) complexes having a chloride ligand, the cyclometallated imine ligand CC5Cl4CHNCH2C6H5, and two methyl groups and the leaving ligand in a mer-geometrical arrangement, [Pt(C∧N)(CH3)2Cl(L)], have been studied as a function of temperature, solvent, and electronic and steric characteristics of the leaving ligands. Although in all cases a limiting dissociative mechanism has been found to apply, where the dissociation of the leaving ligand corresponds to the rate-determining step, the relative values of the different rate constants involved in the process, i.e. dissociation/association of the leaving/entering ligands and the effect of the solvent, change dramatically in the systems studied. The results agree with the previously determined mechanism operating on these substitution reactions, and serve as a very good measure of the strength of the PtL bond, which is influenced by both electronic and steric characteristics. The possible solvent-assisted hydrogen bonding of the leaving ligand with the rest of the molecule is made also evident in the relevant situations. The equilibrium constants detected for some of the systems indicate that, although Pt(IV) complexes are expected to be hard acids and prefer nitrogen to sulfur or phosphorus donors, this is not so. This result is in line with the softening of the nature of the Pt(IV) centres of complexes containing several PtC bonds.