Ligand exchange
If we add ammonia solution to a solution containing hexaaquacopper(II) ions, [Cu(H2O)6]2+, four of the six water ligands are successively replaced by ammonia molecules to form the tetraamminediaquacopper(II) ion, [Cu(NH3)4(H2O)2]2+. The process involved is a dynamic equilibrium with ammonia and water molecules moving into and out of the complex ion.
These changes are described in the following equilibrium equation:
[Cu(H2O)6]2+(aq) + 4NH3(aq) [Cu(NH3)4(H2O)2]2+(aq) + 4H2O(l)
(light blue solution) (dark blue solution)
This type of reaction is called a ligand exchange reaction because some of the water ligands are replaced by ammonia ligands. The change is easy to observe because the solution containing the complex containing ammonia molecules is a much deeper blue than that with the complex containing only water molecule ligands.
If you added water to the solution containing [Cu(NH3)4(H2O)2]2+ ions, what would you expect to see?
The dark blue solution would become lighter as the equilibrium position moves towards the [Cu(H2O)6]2+ ions.
These changes can be seen in the following video clip.
[INSERT LEE_CI_02_vid]
In Magic beakers, Declan Fleming describes a colourful series of changes, several of which involve ligand exchange reactions, while Colin Bakler uses ligand exchange reactions to demonstrate A spectacular reversible reaction.