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Dissociative substitution
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<h2 id="kinetics">Kinetics</h2> <p>Dissociative pathways are characterized by a <a href="/facts/Rate_determining_step/UC0TNxtX">rate determining step</a> that involves release of a ligand from the coordination sphere of the metal undergoing substitution. The concentration of the substituting <a href="/facts/Nucleophile/xYccoaJQ">nucleophile</a> 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 <a href="/facts/Rate_constant/NcKvaTWP">rate constants</a> of their corresponding intermediate reaction steps: </p> L n M − L ⇌ + L , k − 1 − L , k 1 L n M − ◻ → + L ′ , k 2 L n M − L ′ {\displaystyle {\ce {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'}}} <p>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: </p> Rate = k 1 [ L n M − L ] {\displaystyle {\ce {Rate={{\mathit {k}}_{1}[L_{\mathit {n}}M-L]}}}} <p>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 k 2 [ L ′ ] [ Int ] k − 1 [ L ] [ Int ] + k 2 [ L ′ ] [ Int ] {\displaystyle {\ce {\frac {{\mathit {k}}_{2}[L'][Int]}{{{\mathit {k}}_{-1}[L][Int]}+{\mathit {k}}_{2}[L'][Int]}}}} , which leads us to the overall rate equation: </p> Rate overall = ( k 2 [ L ′ ] [ I n t ] k − 1 [ L ] [ I n t ] + k 2 [ L ′ ] [ I n t ] ) ( k 1 [ L n M − L ] ) = k 1 k 2 [ L ′ ] [ L n M − L ] k − 1 [ L ] + k 2 [ L ′ ] {\displaystyle {\ce {Rate_{overall}}}=\left({\frac {k_{2}[{\ce {L}}'][Int]}{{k_{-1}[{\ce {L}}][Int]}+k_{2}[{\ce {L}}'][Int]}}\right)({k_{1}[{\ce {L_{\mathit {n}}M-L}}]})={\frac {k_{1}k_{2}[{\ce {L}}'][{\ce {L_{\mathit {n}}M-L}}]}{{k_{-1}[{\ce {L}}]}+k_{2}[{\ce {L}}']}}} <p>When [L] is small and negligible, the above complex equation reduces to the simple rate equation that depends on k1 and [LnM-L] only. </p> <h2 id="dissociative-interchange-pathway">Dissociative interchange pathway</h2> <p>Interchange pathways apply to <a href="/facts/Substitution_reaction/zpqsbWAo">substitution reactions</a> where <a href="/facts/Reaction_intermediate/BjD9HAHt">intermediates</a> are not observed, which is more common than pure dissociative pathways. If the <a href="/facts/Reaction_rate/Prwvxq0y">reaction rate</a> is insensitive to the nature of the attacking <a href="/facts/Nucleophile/xYccoaJQ">nucleophile</a>, the process is called <i>dissociative interchange</i>, abbreviated <i>I</i>d. An illustrative process comes from the "<a href="/facts/Anation/Da4UTgy2">anation</a>" (reaction with an anion) of cobalt(III) complexes:<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> </p> [ Co ( NH 3 ) 5 ( H 2 O ) ] 3 + + SCN − ↽ − − ⇀ { [ Co ( NH 3 ) 5 ( H 2 O ) ] , NCS } 2 + {\displaystyle {\ce {[Co(NH3)5(H2O)]^3+ + SCN- <=> \{[Co(NH3)5(H2O)], NCS \}^2+}}} { [ Co ( NH 3 ) 5 ( H 2 O ) ] , NCS } 2 + ↽ − − ⇀ [ Co ( NH 3 ) 5 NCS ] 2 + + H 2 O {\displaystyle {\ce {\{[Co(NH3)5(H2O)], NCS \}^2+ <=> [Co(NH3)5NCS]^2+ + H2O}}} <h2 id="water-exchange">Water exchange</h2> <p>The exchange between bulk and <a href="/facts/Coordinated_water/sbaKJNJX">coordinated water</a> is of fundamental interest as a measure of the intrinsic kinetic <a href="/facts/Lability/Gg1wYBfG">lability</a> of metal ions. This rate is relevant to toxicity, <a href="/facts/Catalysis/LPBS7TQC">catalysis</a>, <a href="/facts/Magnetic_resonance_imaging/Vh9yG9qq">magnetic resonance imaging</a>, and other effects. For octahedral mono- and dicationic <a href="/facts/Aquo_complex/eKudHVQy">aquo complexes</a>, these exchange processes occur via an interchange pathway that has more or less dissociative character.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> Rates vary by a factor of 1018, [Ir(H2O)6]3+ being the slowest and [Na(H2O)6]+ being one of the fastest for octahedral complexes. Charge has a significant influence on these rates but non-electrostatic effects are also important. </p> <h2 id="sn1cb-mechanism">Sn1CB mechanism</h2> <p>The rate for the <a href="/facts/Hydrolysis/JTvwIHPs">hydrolysis</a> of cobalt(III) ammine (NH3-containing) halide complexes are deceptive, appearing to be associative but proceeding by a pathway that is dissociative in character. The hydrolysis of [Co(NH3)5Cl]2+ 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 NH3 ligand to give the <a href="/facts/Conjugate_base/hohgQ07M">conjugate base</a> of the starting complex, i.e., [Co(NH3)4(NH2)Cl]+. In this mono<a href="/facts/Cation/wrbwhF3z">cation</a>, the chloride spontaneously dissociates from this conjugate base of the starting complex. This pathway is called the <a href="/facts/Sn1CB_mechanism/8bEvI7Ay">Sn1CB mechanism</a>. </p> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Basolo, F.; Pearson, R. G. "Mechanisms of Inorganic Reactions." John Wiley: New York: 1967. ISBN 0-471-05545-X <a href="/wiki/Fred_Basolo" target="_blank">/wiki/Fred_Basolo</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>R. G. Wilkins "Kinetics and Mechanism of Reactions of Transition Metal Complexes," 2nd Edition, VCH, Weinheim, 1991. ISBN 1-56081-125-0 <a href="/wiki/ISBN_(identifier)" target="_blank">/wiki/ISBN_(identifier)</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>G. L. Miessler and D. A. Tarr “Inorganic Chemistry” 3rd Ed, Pearson/Prentice Hall. ISBN 0-13-035471-6. <a href="/wiki/ISBN_(identifier)" target="_blank">/wiki/ISBN_(identifier)</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Helm, Lothar; Merbach, André E. (2005). "Inorganic and Bioinorganic Solvent Exchange Mechanisms". Chemical Reviews. 105: 1923–1959. doi:10.1021/cr030726o. PMID 15941206. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> </ol>