Quantum capacitance: Difference between revisions

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{{redirect|Dissociative reaction|the disorder formerly known as dissociative reaction|dissociative identity disorder}}


'''Dissociative substitution''' describes a pathway by which compounds interchange [[ligand]]s.  The term is typically applied to [[Coordination chemistry|coordination]] and [[Organometallic chemistry|organometallic]] complexes, but resembles the [[SN1 reaction|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 reaction|Sn2 pathway]].  Intermediate pathways exist between the pure dissociative and pure associative pathways, these are called [[interchange mechanism]]s.<ref>[[Fred Basolo|Basolo]], F.; Pearson, R. G. "Mechanisms of Inorganic Reactions." John Wiley: New York: 1967. ISBN 0-471-05545-X</ref><ref>R. G. Wilkins "Kinetics and Mechanism of Reactions of Transition Metal Complexes," 2nd Edition, VCH, Weinheim, 1991. ISBN 1-56081-125-0</ref>


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.
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==Kinetics==
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:
 
[[File:General reaction with dissociative mechanism.png|center|400px|]]
 
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:
 
:<math> Rate = {k_1 [L_nM-L]}</math>
 
However, in some cases, the back reaction (k<sub>-1</sub>) becomes important, and [L'] can exert an effect on the overall rate of reaction. The backward reaction k<sub>-1</sub> therefore competes with the second forward reaction (k<sub>2</sub>), thus the fraction of intermediate (denoted as "Int") that can react with L' to form the product is given by the expression <math> \frac{k_2[L'][Int]}{k_-1[L][Int]+k_2[L'][Int]}</math>, which leads us to the overall rate equation:
 
:<math> Rate_{overall} = (\frac{k_2[L'][Int]}{k_{-1}[L][Int]+k_2[L'][Int]})({k_1 [L_nM-L]}) = \frac{k_1k_2[L'][L_nM-L]}{k_{-1}[L]+k_2[L']}</math>
 
When [L] is small and negligible, the above complex equation reduces to the simple rate equation that depends on k<sub>1</sub> and [L<sub>n</sub>M-L] only.
 
==Dissociative interchange pathway==
Interchange pathways apply to substitution reactions where intermediates are not observed, which is more common than pure dissociative pathways.  If the reaction rate is insensitive to the nature of the attacking nucleophile, the process is called dissociative interchange, abbreviated ''I''<sub>d</sub>.  An illustrative process comes from the "anation" (reaction with an anion) of cobalt(III) complexes:<ref>G. L. Miessler and D. A. Tarr “Inorganic Chemistry” 3rd Ed, Pearson/Prentice Hall. ISBN 0-13-035471-6.</ref>
::[Co(NH<sub>3</sub>)<sub>5</sub>(H<sub>2</sub>O)]<sup>3+</sup> + [[Thiocyanate|SCN<sup>-</sup>]] {{eqm}} {[Co(NH<sub>3</sub>)<sub>5</sub>(H<sub>2</sub>O)], NCS}<sup>2+</sup>
::{[Co(NH<sub>3</sub>)<sub>5</sub>(H<sub>2</sub>O)], NCS}<sup>2+</sup> {{eqm}} [Co(NH<sub>3</sub>)<sub>5</sub>NCS]<sup>2+</sup> + H<sub>2</sub>O
 
==Water exchange==
The exchange between bulk and coordinated water is of fundamental interest as a measure of the intrinsic kinetic lability of metal ions.  This rate is relevant to toxicity, catalysis, [[magnetic resonance imaging]], and other effects. For octahedral mono- and dicationic [[aquo complex]]es, these exchange processes occur via an interchange pathway that has more or less dissociative character.<ref>Lothar Helm, André E. Merbach "Inorganic and Bioinorganic Solvent Exchange Mechanisms" Chemical Reviews 2005, volume 105, 1923-1959.  {{DOI|10.1021/cr030726o}}</ref> Rates vary by a factor of 10<sup>18</sup>, [Ir(H<sub>2</sub>O)<sub>6</sub>]<sup>3+</sup> being the slowest and [Na(H<sub>2</sub>O)<sub>6</sub>]<sup>+</sup> being one of the fastest for octahedral complexes.  Charge has a significant influence on these rates but non-electrostatic effects are also important.
 
==Sn1CB mechanism==
The rate for the [[hydrolysis]] of cobalt(III) ammine (NH<sub>3</sub>-containing) halide complexes are deceptive, appearing to be associative but proceeding by a pathway that is dissociative in character.  The hydrolysis of [Co(NH<sub>3</sub>)<sub>5</sub>Cl]<sup>2+</sup> follows second order kinetics: the rate increases linearly with concentration of hydroxide as well as the starting complex. Studies show, however, that in the hydroxide deprotonates one NH<sub>3</sub> ligand to give the [[conjugate base]] of the starting complex, i.e., [Co(NH<sub>3</sub>)<sub>4</sub>(NH<sub>2</sub>)Cl]<sup>+</sup>.  In this mono[[cation]], the chloride spontaneously dissociates from this conjugate base of the starting complex. This pathway is called the [[Sn1CB mechanism]].
 
==References==
<references/>
 
[[Category:Organometallic chemistry]]
[[Category:Coordination chemistry]]
[[Category:Chemical reactions]]
[[Category:Reaction mechanisms]]

Latest revision as of 09:22, 30 July 2014


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