HIGH-PRESSURE NMR KINETICS .47. SOLVENT-EXCHANGE MECHANISMS OF NONAQUEOUS SQUARE-PLANAR TETRASOLVATES - A HIGH-PRESSURE H-1-NMR INVESTIGATION

Solvent exchange of square-planar PdS4(2+) complexes, where S = DMA, DMF, MeCN, MeNC, Me2S, Et2S, and Pt(MeNC)4(2+), has been studied as a function of temperature (average temperature range of 40 K) and pressure (0.1-200 MPa) by H-1 NMR line-broadening studies in diluents CD3NO2 and CD3CN. The exchange rates span several orders of magnitude-from k2(298) = 5 m-1 s-1 for Pd(Et2S)4(2+) to 1.06 x 10(6) m-1 s-1 for Pd(MeNC)4(2+)-as a result of large changes in ligand nucleophilicity and of differing encumbrance of ligand and complex. Nevertheless, the volume of activation remains consistently negative with values ranging from -0.1 to -11.6 cm3 mol-1 for Pd(MeCN)4(2+) to Pd(Et2S)4(2+), respectively. In combination with the negative activation entropies obtained, these results indicate that regardless of the nature of the ligand the mechanism of exchange remains associative in nature.