Eigen-Wilkins Mechanism
The Eigen-Wilkins mechanism, named after the German chemist Manfred Eigen and R. G. Wilkins,[1] is a mechanism and rate law in coordination chemistry governing the associative substitution reactions of octahedral complexes. It was discovered for substitution by ammonia of a chromium-(III) hexaaqua complex.[2][3] 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 while the subsequent dissociation to form product P is governed by a rate constant k:
ML6 + Y 15px ML6 → P
Derivation
A simple derivation of the Eigen-Wilkins rate law follows:[4]
[ML6-Y] = KE[ML6][Y]
[ML6-Y] = [M]tot - [ML6]
rate = k[ML6-Y]
rate = kKE[Y][ML6]
Leading to the final form of the rate law,
rate = kKE[Y][M]tot / (1 + KE[Y])
Eigen-Fuoss Equation
A further insight into the pre-equilibrium step and its equilibrium constant KE comes from the Fuoss-Eigen equation proposed independently by Eigen and R. M. Fuoss:
KE = (4πa3/3) x NAexp(-V/RT)
Where a represents the minimum distance of approach between complex and ligand in solution, NA is the Avogadro constant, R is the gas constant and T is the reaction temperature. V is the Coulombic potential energy of the ions at that distance:
V = z1z2e2/4πaε
Where z is the charge number of each species and ε is the vacuum permittivity.
A typical value for KE is 20.2 dm3mol-1 for neutral particles at a distance of 200 pm.[5]
Results
The result of the rate law is that at high concentrations of Y, the rate approximates k[M]tot while at low concentrations the result is kKE[M]tot[Y].
The Eigen-Fuoss equation shows that higher values of KE (and thus a faster pre-equilibrium) are obtained for large, oppositely-charged ions in solution.
References
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<references />, or <references group="..." />- ↑ M. Eigen, R. G. Wilkins: Mechanisms of Inorganic Reactions. In: Advances in Chemistry Series. Nr. 49, 1965, S. 55. American Chemical Society, Washington, D. C.
- ↑ Basolo, F.; Pearson, R. G. "Mechanisms of Inorganic Reactions." John Wiley and Son: New York: 1967. ISBN 047105545X
- ↑ R. G. Wilkins "Kinetics and Mechanism of Reactions of Transition Metal Complexes," 2nd Edition, VCH, Weinheim, 1991. ISBN 1-56081-125-0
- ↑ G. L. Miessler and D. A. Tarr “Inorganic Chemistry” 3rd Ed, Pearson/Prentice Hall publisher, ISBN 0-13-035471-6.
- ↑ Atkins, P. W. (2006). Shriver & Atkins inorganic chemistry. 4th ed. Oxford: Oxford University Press
