Difference Between Associative and Dissociative Mechanism

The key difference between associative and dissociative mechanism is that the associative mechanisms are characterized by the binding of the attacking nucleophile to give a discrete and detectable intermediate followed by the loss of another ligand whereas the dissociative mechanisms are characterized by a rate-determining step that involves the release of a ligand from the coordination sphere of the metal that undergoes the substitution.

The two mechanisms, associate and dissociative mechanism are involved in the organic-chemical synthesis reactions where substitution occur. Therefore, we can name these reaction mechanisms as associative substitution and dissociative substitution.

CONTENTS

1. Overview and Key Difference
2. What is Associative Mechanism
3. What  is Dissociative Mechanism
4. Side by Side Comparison – Associative vs Dissociative Mechanism in Tabular Form
5. Summary

What is Associative Mechanism?

An associative mechanism or associative substitution is a type of organic chemical reaction in which the ligands interchange between molecules, and thus, binding a new ligand to the coordination sphere. It is a pathway by which compounds interchange ligands. Typically, this term is applied to coordination complexes and organometallic complexes. Furthermore, this reaction mechanism resembles the SN2 mechanism in organic chemistry. The opposite mechanism of this associative mechanism is the dissociative mechanism.

Figure 01: Associative Mechanism

Moreover, we can characterize an associative mechanism by the binding of the attacking nucleophile to give a discrete, detectable intermediate followed by the loss of another ligand. The coordination complexes that can undergo this substitution mechanism are coordinatively unsaturated compounds or contain a ligand that can change its bonding to the metal. Examples for associative mechanisms include sixteen electron-square planar metal complexes such as Vaska’s complex.

What is Dissociative Mechanism?

A dissociative mechanism or dissociative substitution is a type of organic chemical reaction in which the ligands interchange between molecules, releasing a ligand from the coordination sphere. It is a pathway in which the compounds interchange ligands. The opposite process of this mechanism is the associative substitution mechanism. Typically, this term is applied to coordination complexes and organometallic complexes. This type of mechanisms resembles the SN1 pathway in organic chemistry.

Moreover, the complexes that can undergo dissociative substitution include the coordinatively saturated compounds having octahedral molecular geometry. Also, in this type of reactions, the entropy of activation is characteristically positive, indicating that the disorder of the reacting system increases in the rate-determining step.

What is the Difference Between Associative and Dissociative Mechanism?

The key difference between associative and dissociative mechanism is that the associative mechanisms are characterized by the binding of the attacking nucleophile to give a discrete and detectable intermediate followed by the loss of another ligand whereas the dissociative mechanisms are characterized by a rate-determining step that involves the release of a ligand from the coordination sphere of the metal that is undergoing the substitution. In brief, associative mechanisms involve in binding a new ligand to the complex compound whereas dissociative mechanisms involve in releasing a ligand from the complex compound. Moreover, the associative mechanism involves coordinatively unsaturated compounds while dissociative mechanism involves coordinatively saturated compounds.

The following table summarizes the differences between associative and dissociative mechanism in more detail.

Summary – Associative vs Dissociative Mechanism

The terms associative and dissociative mechanisms are used in organic synthesis applications. The key difference between associative and dissociative mechanism is that the associative mechanisms are characterized by the binding of the attacking nucleophile to give a discrete and detectable intermediate followed by the loss of another ligand whereas the dissociative mechanisms are characterized by a rate-determining step that involves the release of a ligand from the coordination sphere of the metal that is undergoing the substitution.

Reference:

1. “25: Associative Mechanism.” Chemistry LibreTexts, Libretexts, 15 Aug. 2020, Available here.

Image Courtesy:

1. “AssveRxn” By Smokefoot – Own work (Public Domain) via Commons Wikimedia

ncG1vNJzZmivp6x7pbXFn5yrnZ6YsqOx07CcnqZemLyue8OinZ%2Bdopq7pLGMm5ytr5Wau26t0qymnKGRqba3sYyapZ1llJ7AtLvCopitoaaaeq6xwqGYp6Gjonw%3D