Question

List and describe three pieces of data that support the fact that octahedral complexes undergo dissociative substitution.

Answer 1

Hi,

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. 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. Intermediate pathways exist between the pure dissociative and pure associative pathways, these 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.

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 k₁, k_-1 and k₂, which are the rate constants of their corresponding intermediate reaction steps:

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:

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 , 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 [L_nM-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_d. An illustrative process comes from the "anation" (reaction with an anion) of cobalt(III) complexes:

[Co(NH₃)₅(H₂O)]³⁺ + SCN^? {[Co(NH₃)₅(H₂O)], NCS}²⁺

{[Co(NH₃)₅(H₂O)], NCS}²⁺ [Co(NH₃)₅NCS]²⁺ + H₂O

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http://www.mpilkington.com/Lecture_12.pdf