Ligand substitution is a process in which one ligand (a molecule or ion attached to a metal ion in a complex) is replaced by another ligand. In other words, a ligand “swaps places” with a different one. This is a common type of reaction in coordination chemistry.
There are different ways this can happen:
- Associative mechanism – The new ligand attaches to the metal first, temporarily increasing the number of ligands around the metal, and then the old ligand leaves. This is common in square planar complexes.
- Dissociative mechanism – The original ligand leaves first, creating an empty spot for the new ligand to attach. This is common in octahedral complexes.
- Interchange mechanism – The old ligand leaves and the new ligand attaches at almost the same time.
Factors that influence ligand substitution include:
- The type of metal ion (its size, charge, and electronic configuration).
- The type of ligands (some ligands bind more strongly than others).
- Steric effects (large ligands can block substitution).
- The solvent, which can stabilize or destabilize intermediates.
A simple example is when a water molecule replaces another ligand in a metal complex, changing the complex’s properties.