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The kinetics of anation of the hydroxopentaaquarhodium(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 (AH * = 87.5 kJmol-1 and AS s = -52.3 J K-1 tool-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 (Ia) mechanism with the prior formation of an outer-sphere association complex. Solvent polarity effects lead to the same conclusion.

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