Dissociative substitution

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Dissociative substitution

Dissociative substitution describes a pathway by which compounds interchange ligands. The term is typically applied to coordination and organometallic complexes, but resembles the Sn1 mechanism in organic chemistry. The opposite pathway is associative substitution, being analogous to Sn2 pathway. Intermediate pathways exist between the pure dissociative and pure associative pathways, these are called interchange mechanisms.[1][2]

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. An intermediate of reduced coordination number can be detected. 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 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 Id. An illustrative process comes from the "anation" (reaction with an anion) of cobalt(III) complexes:[3]

[Co(NH3)5(H2O)]3+ + SCN- $\overrightarrow{\leftarrow}$ {[Co(NH3)5(H2O)], NCS}2+
{[Co(NH3)5(H2O)], NCS}2+ $\overrightarrow{\leftarrow}$ [Co(NH3)5NCS]2+ + H2O

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 effects, and others. Many metal mono- and dications form octahedral aquo complexes. Most of these exchange processes occur via an interchange pathway that has more or less dissociative character.[4] 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.

Sn1CB mechanism

The rate for the hydrolysis 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 conjugate base of the starting complex, i.e., [Co(NH3)4(NH2)Cl]+. In this monocation, the chloride spontaneously dissociates from this conjugate base of the starting complex. This pathway is called the Sn1CB mechanism.

References

1. ^ Basolo, F.; Pearson, R. G. "Mechanisms of Inorganic Reactions." John Wiley: New York: 1967. ISBN 047105545X
2. ^ R. G. Wilkins "Kinetics and Mechanism of Reactions of Transition Metal Complexes," 2nd Edition, VCH, Weinheim, 1991. ISBN 1-56081-125-0
3. ^ G. L. Miessler and D. A. Tarr “Inorganic Chemistry” 3rd Ed, Pearson/Prentice Hall. ISBN 0-13-035471-6.
4. ^ Lothar Helm, André E. Merbach "Inorganic and Bioinorganic Solvent Exchange Mechanisms" Chemical Reviews 2005, volume 105, 1923-1959. doi:10.1021/cr030726o

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