Associative Mechanism vs. Dissociative Mechanism: What's the Difference?
Edited by Aimie Carlson || By Janet White || Published on September 12, 2024
An associative mechanism involves the addition of a ligand to a metal complex before the departure of another ligand, while a dissociative mechanism involves the loss of a ligand from a metal complex before the addition of another ligand.
Key Differences
In an associative mechanism, a ligand approaches and attaches to a metal center, forming an intermediate with a higher coordination number before one of the original ligands departs. In a dissociative mechanism, one ligand first detaches from the metal center, creating a coordination site that is subsequently filled by a new ligand.
The associative mechanism is characterized by a rate-determining step involving the association of a ligand, leading to an increase in coordination number. The dissociative mechanism, on the other hand, features a rate-determining step where a ligand dissociates, resulting in a temporary decrease in coordination number.
Associative mechanisms are often observed in square planar and octahedral complexes where the incoming ligand can easily approach the metal center. Dissociative mechanisms are more common in tetrahedral complexes where ligand departure creates space for the incoming ligand.
The choice between associative and dissociative mechanisms depends on various factors, including the nature of the metal, the ligands involved, and the overall geometry of the complex. The mechanism influences the kinetics and the stereochemistry of the reaction.
Understanding the distinction between associative and dissociative mechanisms is crucial in coordination chemistry, as it helps in predicting reaction pathways, designing catalysts, and interpreting experimental data.
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Comparison Chart
Ligand Addition
Occurs before ligand departure
Occurs after ligand departure
Coordination Number
Increases temporarily
Decreases temporarily
Common in
Square planar, octahedral complexes
Tetrahedral complexes
Rate-Determining Step
Association of a ligand
Dissociation of a ligand
Stereochemistry
Often retains stereochemistry
Can lead to stereochemical changes
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Associative Mechanism and Dissociative Mechanism Definitions
Associative Mechanism
A reaction pathway where a ligand adds to a metal complex before another ligand leaves.
The substitution reaction in square planar complexes often follows an associative mechanism.
Dissociative Mechanism
A mechanism where a catalyst loses a ligand or active site before forming a product.
Some hydrolysis reactions catalyzed by metal ions proceed through a dissociative mechanism.
Associative Mechanism
The process by which a ligand binds to a receptor or active site.
The binding of oxygen to hemoglobin follows an associative mechanism.
Dissociative Mechanism
The process by which a ligand dissociates from a receptor or active site.
The release of carbon dioxide from hemoglobin follows a dissociative mechanism.
Associative Mechanism
The process where molecules come together to form a larger complex.
The formation of a transition state in a chemical reaction can be described by an associative mechanism.
Dissociative Mechanism
The process where a complex breaks down into smaller components.
The dissociation of a substrate from an enzyme's active site can be described by a dissociative mechanism.
Associative Mechanism
A mechanism where monomers add to a growing polymer chain in a stepwise manner.
The polymerization of ethylene via a coordination catalyst typically involves an associative mechanism.
Dissociative Mechanism
A reaction pathway where a ligand leaves a metal complex before another ligand adds.
The substitution reaction in tetrahedral complexes often follows a dissociative mechanism.
Associative Mechanism
A mechanism where a catalyst forms an intermediate complex with a reactant before the product is formed.
Many enzymatic reactions proceed through an associative mechanism.
Dissociative Mechanism
A mechanism where a polymer chain loses a monomer unit before adding another.
The depolymerization of polystyrene occurs via a dissociative mechanism.
FAQs
What is a dissociative mechanism in chemistry?
A dissociative mechanism is a reaction pathway where a ligand leaves a metal complex before another ligand adds, decreasing the coordination number temporarily.
What types of complexes commonly exhibit associative mechanisms?
Associative mechanisms are commonly observed in square planar and octahedral complexes.
What is the rate-determining step in an associative mechanism?
The rate-determining step in an associative mechanism is the association of a ligand to the metal complex.
How do associative and dissociative mechanisms influence stereochemistry?
Associative mechanisms often retain stereochemistry, while dissociative mechanisms can lead to stereochemical changes.
What types of complexes commonly exhibit dissociative mechanisms?
Dissociative mechanisms are more common in tetrahedral complexes.
What is the rate-determining step in a dissociative mechanism?
The rate-determining step in a dissociative mechanism is the dissociation of a ligand from the metal complex.
How do ligand properties affect the choice of mechanism?
The size, charge, and electronic properties of ligands can influence whether a reaction follows an associative or dissociative mechanism.
Are dissociative mechanisms more common in certain types of reactions?
Dissociative mechanisms are more common in substitution reactions involving tetrahedral complexes and in some decomposition reactions.
What is an associative mechanism in chemistry?
An associative mechanism is a reaction pathway where a ligand adds to a metal complex before another ligand departs, increasing the coordination number temporarily.
How do associative and dissociative mechanisms differ in terms of ligand addition?
In associative mechanisms, ligand addition occurs before ligand departure, while in dissociative mechanisms, ligand departure occurs before ligand addition.
Can catalysts influence the choice of mechanism?
Yes, catalysts can influence the choice of mechanism by stabilizing certain intermediates or transition states, thereby favoring either associative or dissociative pathways.
How do associative and dissociative mechanisms affect the coordination number?
Associative mechanisms temporarily increase the coordination number, while dissociative mechanisms temporarily decrease it.
Can a reaction switch between associative and dissociative mechanisms?
Yes, a reaction can switch between associative and dissociative mechanisms depending on conditions such as temperature, pressure, and the nature of the ligands.
Are associative mechanisms more common in certain types of reactions?
Associative mechanisms are more common in substitution reactions involving square planar and octahedral complexes.
Are there any computational methods to predict the mechanism?
Computational methods such as density functional theory (DFT) and molecular dynamics simulations can be used to predict and analyze associative and dissociative mechanisms.
Can solvent effects influence the mechanism?
Yes, solvent effects can influence the mechanism by stabilizing intermediates or transition states, thus favoring either associative or dissociative pathways.
How do temperature and pressure affect the mechanism?
Temperature and pressure can affect the activation energy and stability of intermediates, influencing the preference for associative or dissociative mechanisms.
How does the metal center influence the mechanism?
The size, charge, and electronic configuration of the metal center can determine the preference for an associative or dissociative mechanism.
How are associative and dissociative mechanisms studied experimentally?
They are studied using techniques such as spectroscopy, kinetics measurements, and computational methods to analyze reaction intermediates and rates.
How do associative and dissociative mechanisms impact reaction kinetics?
The choice of mechanism impacts reaction kinetics by determining the rate-determining step and the overall rate law of the reaction.
About Author
Written by
Janet WhiteJanet White has been an esteemed writer and blogger for Difference Wiki. Holding a Master's degree in Science and Medical Journalism from the prestigious Boston University, she has consistently demonstrated her expertise and passion for her field. When she's not immersed in her work, Janet relishes her time exercising, delving into a good book, and cherishing moments with friends and family.
Edited by
Aimie CarlsonAimie Carlson, holding a master's degree in English literature, is a fervent English language enthusiast. She lends her writing talents to Difference Wiki, a prominent website that specializes in comparisons, offering readers insightful analyses that both captivate and inform.
