KINETICS AND MECHANISM OF ANATION OF HEXAAQUOCHROMIUM(III) BY 2-AMINOPYRIDINE

The kinetics of substitution of aqua ligands from the hexaaquochromium(III) ion by 2-aminopyridine (2-ampyH+) in aqueous medium has been studied spectrophotometrically in the 40-55-degrees-C range. The rate law involving the outer sphere complex formation has been established at pH 2.7 as d[Cr(2-ampy)2(H2O)43+]/dt = k(a)K(osc)[Cr(H2O)63+]tot[2-ampyH+]/1+K(osc)[2-ampyH+] where K(osc) is the outer sphere complex equilibrium constant and k(a) is the rate constant for conversion of the outer sphere complex to the inner sphere complex. The reaction rate is pH dependent in the range 2.0-5.0. Activation parameters have been calculated and compared with water exchange and the substitution of the hexaaquochromium(III) ion. A mechanism involving outer sphere association between the two reactant species followed by transformation of this outer sphere complex into the product by associative path (I(a)) is suggested. The mechanism is supported by the activation parameters for the reaction in 20% H2O-EtOH medium.