Editing Substitution reaction (section)

==Inorganic and organometallic chemistry==
While it is common to discuss substitution reactions in the context of organic chemistry, the reaction is generic and applies to a wide range of compounds.  Ligands in coordination complexes are susceptible to substitution.  Both associative and dissociative mechanisms have been observed.<ref>Basolo, F.; Pearson, R. G. "Mechanisms of Inorganic Reactions." John Wiley and Son: New York: 1967. {{ISBN|0-471-05545-X}}</ref><ref>{{cite book |first=R. G. |last=Wilkins |title=Kinetics and Mechanism of Reactions of Transition Metal Complexes |url=https://archive.org/details/kineticsmechanis00wilk_0 |url-access=registration |edition=2nd |publisher=VCH |location=Weinheim |year=1991 |isbn=1-56081-125-0 }}</ref>

[[Associative substitution]], for example, is typically applied to [[Organometallic chemistry|organometallic]] and [[coordination complex]]es, but resembles the [[SN2 reaction|Sn2 mechanism]] in [[organic chemistry]].  The opposite pathway is [[dissociative substitution]], being analogous to the [[SN1 reaction|Sn1 pathway]]. 

Examples of associative mechanisms are commonly found in the chemistry of 16e [[Square planar molecular geometry|square planar]] metal complexes, e.g. [[Vaska's complex]] and [[Potassium tetrachloroplatinate|tetrachloroplatinate]]. The [[rate law]] is governed by the [[Associative substitution#Eigen-Wilkins mechanism|Eigen–Wilkins Mechanism]].
[[File:AssveRxn.png|520px|center]]

[[Dissociative substitution]] resembles the [[SN1 reaction|S<sub>N</sub>1 mechanism]] in organic chemistry.  This pathway can be well described by the [[cis effect|''cis'' effect]], or the labilization of CO ligands in the ''cis'' position. Complexes that undergo dissociative substitution are often [[coordinative unsaturation|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 k<sub>1</sub>, k<sub>−1</sub> and k<sub>2</sub>, which are the [[rate constant]]s of their corresponding intermediate reaction steps:

:<chem>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'</chem>
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:

:<chem> Rate = {\mathit k_1 [L_\mathit{n}M-L]}</chem>