Associative Substitution - Eigen-Wilkins Mechanism

Eigen-Wilkins Mechanism

The Eigen-Wilkins mechanism, named after chemists Manfred Eigen and R. G. Wilkins, is a mechanism and rate law in coordination chemistry governing associative substitution reactions of octahedral complexes. It was discovered for substitution by ammonia of a chromium-(III) hexaaqua complex. The key feature of the mechanism is an initial rate-determining pre-equilibrium to form an encounter complex ML₆-Y from reactant ML₆ and incoming ligand Y. This equilibrium is represented by the constant K_E:

ML₆ + Y ML₆-Y

The subsequent dissociation to form product is governed by a rate constant k:

ML₆-Y → ML₅Y + L

A simple derivation of the Eigen-Wilkins rate law follows:

= K_E

= _tot -

rate = k

rate = kK_E

Leading to the final form of the rate law, using the steady-state approximation (d / dt = 0),

rate = kK_Etot / (1 + K_E)