Associative substitution
Associative substitution describes a pathway by which compounds interchange ligands. The terminology is typically applied to organometallic and coordination complexes, but resembles the Sn2 mechanism in organic chemistry. The opposite pathway is dissociative substitution, being analogous to the Sn1 pathway. Intermediate pathways exist between the pure associative and pure dissociative pathways, these are called interchange mechanisms.[1][2]
Associative pathways are characterized by
Examples of associative mechanisms are commonly found in the chemistry of 16e
Associative interchange pathway
In many substitution reactions, well-defined intermediates are not observed, when the rate of such processes are influenced by the nature of the entering ligand, the pathway is called associative interchange, abbreviated Ia.[3] Representative is the interchange of bulk and coordinated water in [V(H2O)6]2+. In contrast, the slightly more compact ion [Ni(H2O)6]2+ exchanges water via the Id.[4]
Effects of ion pairing
Polycationic complexes tend to form ion pairs with anions and these ion pairs often undergo reactions via the Ia pathway. The electrostatically held nucleophile can exchange positions with a ligand in the first coordination sphere, resulting in net substitution. An illustrative process comes from the "anation" (reaction with an anion) of chromium(III) hexaaquo complex:
- [Cr(H2O)6]3+ + SCN− ⇌ {[Cr(H2O)6], NCS}2+
- {[Cr(H2O)6], NCS}2+ ⇌ [Cr(H2O)5NCS]2+ + H2O
Special ligand effects
In special situations, some ligands participate in substitution reactions leading to associative pathways. These ligands can adopt multiple motifs for binding to the metal, each of which involves a different number of electrons "donated." A classic case is the
SN1cB mechanism
The rate for the
Eigen-Wilkins mechanism
The Eigen-Wilkins mechanism, named after chemists Manfred Eigen and R. G. Wilkins,[5] 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.[6][7] 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:[8]
- [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, using the steady-state approximation (d[ML6-Y] / dt = 0),
- 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 = (4πa3/3000) x NAexp(-V/RT)
Where a represents the minimum distance of approach between complex and ligand in solution (in cm), NA is the
- V = z1z2e2/4πaε
Where z is the charge number of each species and ε is the vacuum permittivity.
A typical value for KE is 0.0202 dm3mol−1 for neutral particles at a distance of 200 pm.[9] 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
- ISBN 0-471-05545-X.
- ISBN 1-56081-125-0.
- ISBN 0-13-035471-6.
- PMID 15941206.
- ^ 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.
- ISBN 047105545X
- ISBN 1-56081-125-0
- ISBN 0-13-035471-6.
- ^ Atkins, P. W. (2006). Shriver & Atkins inorganic chemistry. 4th ed. Oxford: Oxford University Press