On the use of "fast kinetics" to determine the mechanism of ligand substitution at a solvated transition-metal intermediate

Substitution of the weakly-coordinated solvent molecule at a solvated transition-metal intermediate is frequently investigated by "fast kinetic" methods. In typical experiments, the kinetics of the reaction are determined by following the time dependence of the changes in the reaction mixture's UV-visible or infrared spectrum following photolytic creation of the intermediate. We consider the two limiting mechanisms (associative and dissociative), as well as the case of competition between them, and show that under typical "fast kinetics" experimental conditions, the different mechanisms are kinetically indistinguishable.