Colour Changes in Transition Metal Ions

Understanding the colour changes in transition metal ions is key. The colour changes in various chemical reactions are due to the different oxidation states of transition metals. Here are some points:

• Transition metal ions are often coloured because they contain unpaired d-block electrons.
• The colour of the complex ion depends on several factors like the nature of the transition metal ion, its oxidation state and the type of ligands attached to it.
• An electron can move from one d-orbital to another of higher energy, by absorbing a certain amount of energy which corresponds to a particular wavelength of light. The remainder of the light is transmitted or reflected and this gives the compound its colour.
• For example, transition metal iron (Fe) in its different oxidation states shows diverse colours. Fe2+ ions are light green, while Fe3+ ions are reddish-brown.
• In copper(II) sulphate (CuSO4), the Cu2+ ion imparts a blue colour. When ammonia solution is added, a deep blue precipitate of copper hydroxide forms initially. As more ammonia is added, this precipitate dissolves to form tetraamminecopper(II) ions [Cu(NH3)4(H2O)2]2+, which are a deep blue colour.
• Remember that not all transition metal ions are coloured (for instance, Sc3+ or Zn2+), because they either have full or empty d sub-shells.
• The correlation between wavelength of light absorbed and colour observed can be remembered using a simple colour wheel. If a substance absorbs a certain colour, the colour observed will be the complementary colour on the wheel.

Remember to relate these colour changes with the observed chemical reactions in your practical work, and use these changes to deduce the different transition metal ions present in any given solution.