FACTORS AFFECTING REACTION PATHWAYS IN NUCLEOPHILIC-SUBSTITUTION REACTIONS ON PLATINUM(II) COMPLEXES - A COMPARATIVE KINETIC AND THEORETICAL-STUDY

Kinetic studies in CH2Cl2 of the displacement of the thioethers from cis-[PtPh2(Et2S)2] with pyridines to yield cis-[PtPh2(py)2] show, in agreement with previous findings, that the rate-determining step is dissociation of a molecule of thioether and the formation of a transient 3-coordinate [Pt(Ph)2(Et2S)] intermediate. At 298.16 K, the rate constant for dissociation k1 = (2.12 +/- 0.1) X 10(-3) S-1, DELTAH double dagger = 99.9 +/- 1 kJ mol-1 and DELTAS double dagger = +39 +/- 4 J mol-I K-1. In contrast, the complex cis-[PtPh2(CO)(Et2S)], in the same reactions and under the same experimental conditions, undergoes substitution of Et2S only through a bimolecular pathway, showing a considerable discrimination among various nucleophiles. In the reaction with pyridine, the second-order rate constant k2 = 30.0 +/- 0.1 M-1 s-1, DELTAH double dagger = 34 +/- 1 kJ mol-1, DELTAS double dagger = -174 +/- 4 J mol-1 K-1. Extended Huckel theory (EHT) and molecular electrostatic potential (MEP) calculations were performed on molecules of the type CiS- [PtCl2S2], CiS- [PtMe2S2], and cis-[PtMe2(CO)S](S = Me2S or Me2SO), searching for a correlation between electronic properties and pathways followed for substitution. The sharp changeover of mechanism (dissociative vs. associative) observed on going from the diaryl disulfide to the diaryl carbonyl sulfide platinum(II) complex, is accounted for by the presence on the latter of a LUMO perpendicular to the plane, suitable to the nucleophilic attack, and by a large positive electronic potential. Dissociation is a combined result of ground-state destabilization and of a concurrent increase of electron density at the metal, preventing the approach of nucleophiles.