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 KE:
- 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:
- = KE
- = tot -
- rate = k
- rate = kKE
Leading to the final form of the rate law, using the steady-state approximation (d / dt = 0),
- rate = kKEtot / (1 + KE)
Read more about this topic: Associative Substitution
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