Studies on the kinetics and mechanism of anation reactions of some six-coordinate complexes of chromium(III) in aqueous solution

Rates of stepwise anation of cis-Cr(ox)(2)(H2O)(2)(-) with SCN-/N-3(-), Cr(acac)(2)(H2O)(2)(+) with SCN- and Cr(atda) (H2O)(2) with SCN- have been investigated in weakly acidic aqueous solutions. Rate constants, k(I) and k(II) for the two steps in each system, are composite as k(x)=k(x)(0) + k(x)(X)[X(-)](x=I, II; X(-)=SCN-, N-3(-)). These rate constants have been evaluated also as the corresponding Delta H double dagger and Delta S double dagger values. The results obtained and the plausible I, mechanism seem to suggest Cr-OOC bond dissociation (hence a strongly negative Delta S double dagger) generating the transition state in each system with outer-sphere association forming the precursor complex in the X(-) dependent paths.