What is the Difference Between Associative and Dissociative Mechanism?

The main difference between associative and dissociative mechanisms lies in the order of events that occur during the formation of a coordination complex. Both mechanisms involve the exchange of ligands in a coordination complex, but they proceed through different pathways:

Associative Mechanism (A):

In this mechanism, the incoming ligand (Y) first binds to the coordination sphere, forming an intermediate with a higher coordination number (MLnXY).
The rate-determining step is the ligand attack, where the new ligand (Y) binds to the coordination sphere.
The overall reaction is represented as: $$\ce{ML{n}X + Y <=> ML{n}XY <=> ML_{n}Y + X}$$.
The geometry of the end product remains the same as the starting complex.

Dissociative Mechanism (D):

In this mechanism, the old ligand (X) first leaves the coordination sphere, forming an intermediate with a lower coordination number (MLn).
The rate-determining step is the bond-breaking process, where the old ligand (X) leaves the coordination sphere.
The overall reaction is represented as: $$\ce{ML{n}X <=> ML{n} + X <=> ML_{n}Y + Y}$$.
The geometry of the end product can be different from the starting complex.

In summary, the associative mechanism involves the incoming ligand binding first, followed by the departure of the old ligand, while the dissociative mechanism involves the old ligand leaving first, followed by the binding of the new ligand. The rate-determining steps, as well as the geometry of the end products, also differ between the two mechanisms.

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What is the Difference Between Associative and Dissociative Mechanism?

Comparative Table: Associative vs Dissociative Mechanism

The table below highlights the differences between associative and dissociative mechanisms:

Feature	Associative Mechanism	Dissociative Mechanism
Description	A type of organic chemical reaction where a new ligand binds to the coordination sphere, replacing an existing ligand.	A type of organic chemical reaction where a ligand breaks its bond with the coordination sphere, creating an intermediate with a lower coordination number.
Reaction Type	Resembles the SN2 mechanism in organic chemistry.	Does not resemble any specific organic reaction mechanism.
Intermediate	The intermediate formed has a higher coordination number than the reactant.	The intermediate formed has a lower coordination number than the reactant.
Ligand Substitution	The incoming ligand becomes part of the complex before the leaving ligand detaches.	The leaving ligand detaches from the complex before the incoming ligand becomes part of the complex.
Coordination Unsaturation	Typically occurs in coordinatively unsaturated complexes (metals with less than 18 electrons).	Can occur in both coordinatively saturated and unsaturated complexes.
Solvent Involvement	The rate of the reaction depends on the solvent's involvement.	The rate of the reaction is not directly affected by the solvent.

Associative mechanisms are involved in coordinative unsaturated complexes and resemble the SN2 mechanism in organic chemistry. In contrast, dissociative mechanisms involve creating an intermediate with a lower coordination number and can occur in both coordinatively saturated and unsaturated complexes.

Guilherme Mazui

Guilherme Mazui is graduated in journalism from the Federal University of Minas Gerais (UFMG) and a master's degree in Communication from the University of São Paulo (USP). In addition, he has experience in advertising writing and has worked as a content editor in several companies.