The π-acceptor effect in the substitution reactions of tridentate N-donor ligand complexes of platinum(II): a detailed kinetic and mechanistic study

The nucleophilic substitution reactions of complexes [Pt{4'-(2"'-CH3-phenyl)-2,2':6',2 ''-terpyridine}Cl]-CF3SO3, [CH3PhPtCl], [Pt{4'-(2"'-CH3-phenyl)-6-(3 ''-isoquinoyl)-2,2'bipyridine}Cl]SbF6, [CH(3)PhisoqPtCl], [Pt{2-(2'-pyridyl)-1,10-phenanthroline}Cl]Cl, [pyPhenPtCl], and [Pt(terpyridine)-Cl](+), [PtCl] with a series of nucleophiles: thiourea (TU), N,N-dimethylthiourea (DMTU), N,N,N,N-tetramethylthiourea (TMTU), I-, Br-, and SCN- were studied in 0.1 M LiCF3SO3 in methanol (in the presence of 10 mM LiCl). The reactivity of the investigated complexes follows the order pyPhenPtCl > PtCl > CH3PhPtCl > CH(3)PhisoqPtCl. The lability of the chloride ligand is dependent on the strength of pi-backbonding properties of the spectator ligands around the platinum centre. The experimental data is strongly supported by DFT calculations. The dependence of the second-order rate constants on concentration of the nucleophiles as well as the large negative values reported for the activation entropy (Delta S-double dagger) confirmed an associative mechanism of substitution.