Kinetics and mechanism of the complexation between hydroxopentaaquochromium(III) and octacyanomolybdate(IV) ions In acidic medium

The reaction between hydroxopentaaquochromium(III) and octacyanomolybdate(IV) was investigated spectrophotometrically and obeyed a 2:1 reactant stoichiometry with respect to formation of the [Cr(H2O)(4)OH](2) Mo(CN)(8) complex. Kinetic studies reveal that the reaction is first order with respect to hydroxopentaaquochromium(III) in the presence of an excess of octacyanomolybdate(IV). The reaction rate increased with an increase in the ionic strength and temperature, and decreased with an increase in hydrogen ion concentration. A mechanism has been proposed based upon ion-pair formation. The results are best accounted for by the Eigen-Tamm mechanism. Anation of [Cr(H2O)(5)OH](2+) is discussed in terms of an associative interchange (I-a) where bond breaking and bond making are equally important. The activation parameters were calculated using Arrhenius's equation.