Ligand exchange in transition metal complexes is the process where one ligand in a complex is replaced by another ligand. This happens because transition metals can form coordinate covalent bonds with different ligands. Here’s a explanation:
1. Mechanism of Ligand Exchange
Ligand exchange can occur mainly by two types of mechanisms:
a) Associative mechanism (A):
- A new ligand attaches to the metal first, temporarily increasing the coordination number.
- Then, one of the original ligands leaves.
- Common for smaller, less crowded metal complexes.
b) Dissociative mechanism (D):
- A ligand leaves the metal first, creating a vacant site.
- The new ligand then attaches to the metal.
- Common for larger, crowded complexes.
Sometimes a combination of both (interchange mechanism, I) occurs.
2. Factors Affecting Ligand Exchange
- Nature of the metal: Transition metals with high positive charge or small size exchange ligands more slowly.
- Nature of the ligand: Ligands that form stronger bonds with the metal are less easily replaced.
- Steric hindrance: Bulky ligands slow down ligand exchange.
- Solvent and temperature: Polar solvents and higher temperatures can increase the rate of exchange.
3. Example in Words
- In an aqueous solution, a water molecule in a metal complex can be replaced by an ammonia molecule:
- Water leaves or ammonia attaches depending on the mechanism.
In short:
Ligand exchange is when one ligand in a transition metal complex is replaced by another, and it occurs via associative, dissociative, or intermediate mechanisms depending on the metal, ligand, and environment.