DISPLACEMENT OF AQUA-LIGANDS FROM THE HYDROXOPENTAAQUARHODIUM(III) ION BY PYRIDINE-2-ALDOXIME - A KINETIC AND MECHANISTIC APPROACH

The kinetics of anation of the hydroxopentaaquarhodium(III) ion by pyr idine-2-aldoxime have been studied spectrophotometrically as a functio n of pH, substrate concentration, entering ligand, temperature and sol vent polarity. The reaction rate increases with an increase in pH, tem perature and ligand concentration, and reaches a limiting value at hig h ligand concentration. The activation parameters (Delta H-not equal = 87.5 kJ mol(-1) and Delta S-not equal = -52.3 J K-1 mol(-1)) have bee n calculated from an Eyring plot and are compared with aqua exchanges in other systems. All the experimental facts lead to an associative in terchange (I-a) mechanism with the prior formation of an outer-sphere association complex. Solvent polarity effects lead to the same conclus ion.