Displacement of aqua ligands from the hydroxopentaaquarhodium(III) ion by pyridine-2-aldoxime: A kinetic and mechanistic approach

Summary The kinetics of anation of the hydroxopentaaqua-rhodium(III) ion by pyridine-2-aldoxime have been studied spectrophotometrically as a function of pH, substrate concentration, entering ligand, temperature and solvent polarity. The reaction rate increases with an increase in pH, temperature and ligand concentration, and reaches a limiting value at high ligand concentration. The activation parameters (H^# = 87.5 kJ mol^–1 and S = –52.3 JK^–1 mol^–1) have been calculated from an Eyring plot and are compared with aqua exchanges in other systems. All the experimental facts lead to an associative interchange (I_a) mechanism with the prior formation of an outer-sphere association complex. Solvent polarity effects lead to the same conclusion.