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 stu died 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(osc)[Cr(H2O)63+]tot[2-ampyH+]/ 1+K(osc)[2-ampyH+] where K(osc) is the outer sphere complex equilibriu m constant and k(a) is the rate constant for conversion of the outer s phere complex to the inner sphere complex. The reaction rate is pH dep endent in the range 2.0-5.0. Activation parameters have been calculate d and compared with water exchange and the substitution of the hexaaqu ochromium(III) ion. A mechanism involving outer sphere association bet ween the two reactant species followed by transformation of this outer sphere complex into the product by associative path (I(a)) is suggest ed. The mechanism is supported by the activation parameters for the re action in 20% H2O-EtOH medium.