Kinetics
Dissociative pathways are characterized by a rate determining step that involves release of a ligand from the coordination sphere of the metal undergoing substitution. The concentration of the substituting nucleophile has no influence on this rate, and an intermediate of reduced coordination number can be detected. The reaction can be described with k1, k-1 and k2, which are the rate constants of their corresponding intermediate reaction steps:
Normally the rate determining step is the dissociation of L from the complex, and does not affect the rate of reaction, leading to the simple rate equation:
However, in some cases, the back reaction (k-1) becomes important, and can exert an effect on the overall rate of reaction. The backward reaction k-1 therefore competes with the second forward reaction (k2), thus the fraction of intermediate (denoted as "Int") that can react with L' to form the product is given by the expression, which leads us to the overall rate equation:
When is small and negligible, the above complex equation reduces to the simple rate equation that depends on k1 and only.
Read more about this topic: Dissociative Substitution