H2 Chemistry 13 - Transition Elements: Ligand Exchange and Colour Changes of Copper(II)

Step 1 - State the problem

Describe and explain the colour changes observed when excess concentrated ammonia is added to an aqueous solution of copper two sulfate.

Step 1 - State the problem

Copper two sulfate solution is pale blue. When dilute ammonia is first added, a pale blue precipitate forms.

Step 1 - State the problem

When excess concentrated ammonia is added, the precipitate dissolves to give a deep blue solution.

Step 1 - State the problem

We need to explain each observation using ligand exchange reactions.

Step 2 - Initial reaction: formation of copper(II) hydroxide

When a small amount of ammonia is added, the ammonia acts as a base.

Step 2 - Initial reaction: formation of copper(II) hydroxide

The hydroxide ions from the ammonia solution react with the hexaaquacopper two ion to form copper two hydroxide, a pale blue precipitate.

Step 2 - Initial reaction: formation of copper(II) hydroxide

This is a deprotonation reaction: the hydroxide ion removes a proton from a coordinated water ligand.

Step 2 - Initial reaction: formation of copper(II) hydroxide

Two water molecules are replaced by two hydroxide ions.

Step 3 - Excess ammonia: ligand exchange to form tetraamminecopper(II)

When excess concentrated ammonia is added, ammonia molecules act as ligands.

Step 3 - Excess ammonia: ligand exchange to form tetraamminecopper(II)

Four of the six water ligands in the hexaaquacopper two ion are replaced by four ammonia molecules.

Step 3 - Excess ammonia: ligand exchange to form tetraamminecopper(II)

The resulting complex is the tetraamminecopper two ion, which has a deep blue colour.

Step 3 - Excess ammonia: ligand exchange to form tetraamminecopper(II)

The two remaining water molecules stay in the axial positions. Ammonia is a stronger ligand than water, so this exchange is thermodynamically favourable.

Step 4 - Explain the colour using d - d transitions

Transition metal complexes are coloured because of d to d electronic transitions.

Step 4 - Explain the colour using d - d transitions

In the copper two ion, the three d orbitals are split by the ligand field into two energy levels.

Step 4 - Explain the colour using d - d transitions

When visible light passes through the solution, a photon is absorbed, promoting an electron from the lower energy d orbitals to the higher energy d orbitals.

Step 4 - Explain the colour using d - d transitions

The colour observed is the complementary colour of the light absorbed. Different ligands cause different splitting energies, which is why the colour changes from pale blue to deep blue.

Step 5 - Summarise and common pitfalls

To summarise: adding dilute ammonia gives a pale blue precipitate of copper two hydroxide. Adding excess ammonia causes ligand exchange, replacing four water molecules with four ammonia molecules to form the deep blue tetraamminecopper two complex.

Step 5 - Summarise and common pitfalls

Common mistake one: writing six ammonia ligands instead of four. Only four water molecules are replaced in the copper ammonia complex.

Step 5 - Summarise and common pitfalls

Common mistake two: saying the colour is due to electron transfer between ligand and metal. For most transition metal complexes at A-level, the colour arises from d to d transitions, not charge transfer.

Step 5 - Summarise and common pitfalls

Always state the ligand, the coordination number, and link the colour to d orbital splitting.