Replacing ligands
Ligand substitution, or ligand exchange, occurs when one or more ligands in a complex are replaced by different ligands.
Substitution can be complete, where all ligands of one type are replaced, or partial, where some of the original ligands remain bonded to the central metal ion.
Water and ammonia are similar in size and are both neutral, monodentate ligands. Exchange between these ligands can therefore occur without changing the coordination number.
Cobalt(II): complete substitution
Ammonia can replace all six water ligands around cobalt(II):
There are six coordinate bonds before and after substitution, so the coordination number remains 6.
Copper(II): incomplete substitution
With copper(II), substitution by ammonia can be incomplete. Four water ligands are replaced while two remain:
The coordination number remains 6 because four ammonia ligands and two water ligands each form one coordinate bond.
| Example | Extent of substitution | Coordination number |
|---|---|---|
| → | Complete | 6 → 6 |
| → | Incomplete | 6 → 6 |
Exam Tip: When deciding whether coordination number changes during ligand substitution, count the coordinate bonds to the central metal rather than simply counting ligand particles.