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Concept
Dissociative substitution
Summary
In chemistry, dissociative substitution describes a reaction pathway by which compounds interchange ligands. The term is typically applied to coordination and organometallic complexes, but resembles the SN1 mechanism in organic chemistry. This pathway can be well described by the cis effect, or the labilization of CO ligands in the cis position. The opposite pathway is associative substitution, being analogous to SN2 pathway. Pathways that are intermediate between the pure dissociative and pure associative pathways are called interchange mechanisms.
Complexes that undergo dissociative substitution are often coordinatively saturated and often have octahedral molecular geometry. The entropy of activation is characteristically positive for these reactions, which indicates that the disorder of the reacting system increases in the rate-determining step.
Dissociative pathways are characterized by a rate determining step that involves release of a ligand from the coordination sphere of the metal undergoing substitution. The concentration of the substituting nucleophile has no influence on this rate, and an intermediate of reduced coordination number can be detected. The reaction can be described with k1, k−1 and k2, which are the rate constants of their corresponding intermediate reaction steps:
L_\mathit{n}M-L [-\mathrm L, k_1][+\mathrm L, k_{-1}] L_\mathit{n}M-\Box ->[+\mathrm L', k_2] L_\mathit{n}M-L'
Normally the rate determining step is the dissociation of L from the complex, and [L'] does not affect the rate of reaction, leading to the simple rate equation:
Rate = {\mathit k_1 [L_\mathit{n}M-L]}
However, in some cases, the back reaction (k−1) becomes important, and [L'] can exert an effect on the overall rate of reaction. The backward reaction k−1 therefore competes with the second forward reaction (k2), thus the fraction of intermediate (denoted as "Int") that can react with L' to form the product is given by the expression \frac{\mathit k_2[L'][Int]}\mathit k_{-1}[L][Int]}+\mathit k_2[L'][Int]}, which leads us to the overall rate equation:
When [L] is small and negligible, the above complex equation reduces to the simple rate equation that depends on k1 and [LnM-L] only.
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