Ligand Substitution Reaction

The molecules attached to the transition metal ion in a complex are called ligands. They can be replaced by other molecules taht are able to donate a pair of electrons. The three most common ligands at A-level are water (H₂O), ammonia (NH₃) and chloride (Cl^-).

Water and ammonia are fairly similar in size and property and so the aqua and ammine complexes usually form the same, octahedral, shape. Chloride (notice that it is the 1- ion) is much larger and, obviously, is negatively charged. This means that it is more likely to form tetrahedral complexes.

It is a good idea to treat ligand substitution as you would equilibria.

If you wish to create an ammine complex, add concentrated ammonia solution.

If you wish to create a chloro complex, add concentrated hydrochloric acid (this is the most soluble of all chlorides and so gives the highest chloride concentration).

If you wish to create the aqua complex, just add loads of water.