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 ML-Y from reactant ML and incoming ligand Y. This equilibrium is represented by the constant K:
:ML + Y  ML-Y 
The subsequent dissociation to form product is governed by a rate constant k:
:ML-Y   → MLY  +  L
A simple derivation of the Eigen-Wilkins rate law follows:
:[ML-Y] = K[ML][Y]
:[ML-Y] = [M] - [ML]
:rate = k[ML-Y]
:rate = kK[Y][ML]
Leading to the final form of the rate law, using the steady-state approximation (d[ML-Y] / dt = 0),
:rate = kK[Y][M] / (1 + K[Y])