Thermodynamic, kinetic and structural studies on the ternary palladium(II)complexes of thioether ligands

Potentiometric, calorimetric, NMR and stopped-flow kinetic studies were per formed on the palladium(II) complexes of thioether and/or nitrogen donor li gands. The ternary systems always contained a tridentate ligand (dien, terp y and dianions of dipeptides, GlyGly, GlyAla and GlyMet) and a monodentate thioether (AcMet). The stability constants of thioether complexes were obta ined by indirect potentiometric measurements using uridine as a competitive ligand. The thermodynamic parameters revealed that selectivity of palladiu m(II) for thioether binding can be significantly influenced by the other do nor atoms around the metal ion. [Pd(terpy)](2+) and [Pd(GlyMet)] had the lo west affinity for thioether binding and it was explained by steric and elec tronic effects. Ternary complexes of nitrogen donors have higher thermodyna mic stability constants than the thioether complexes, but rate constants of the substitution reactions revealed that formation of thioether complexes is the faster reaction. As a consequence, the thermodynamic equilibrium sta te of a multicomponent system is characterized by the coordination of N-don ors, which are formed via the existence of thioether-bonded intermediates. (C) 2000 Elsevier Science Inc. All rights reserved.