Kinetic and Mechanistic Studies of the Interaction of Hydroxopentaaquarhodium(III) Ion with Cyanocobalamin

The substitution reaction of cyanocobalamin with pentaaquarhodium (III) has been studied spectrophotometrically as a function of pH, [Rh(III)]T and [cyanocobalamin]T . The rate of the reaction increases with increase in [Rh(III)]T but decreases with increase in [H+] ion. The activation parameters H#(55.8 ± 3.4) kJ mol−1 and S#(−136 ± 10) J K−1mol−1 have been calculated from the Eyring plots and anation rate constant ks was compared with the aqua exchange and anation rate constants of analogous substitution reactions. G◦, H◦, S◦ for the outer sphere association process have been calculated from kinetic measurement. The mechanistic pathway for the substitution reaction within the outer sphere complex is consistent with an association interchange (Ia) type of phenomenon.