Coloured complexes

Coloured Complexes Overview

• Coloured complexes refer to the complex ions or substances that have transition metals in charge of exhibiting a specific colour.
• These colours arise because the d orbitals in transition metals can absorb light of certain wavelengths resulting in electronic transitions, thereby emitting another colour.

Formation of Coloured Complexes

• Coloured complexes form when transition metals bind with ligands to form coordination complexes.
• When the d-block elements form complexes, the degeneracy (equal energy) of the d orbitals is removed. The energy difference between these two sets of d orbitals allows for the absorption of visible light.
• The absorption of the light lifts an electron from a lower d orbital to a higher one. The complementary colour to the absorbed light gives the colour of the complex.

Colours and Wavelengths

• Each colour corresponds to a specific energy difference, and hence a specific wavelength of light. Different metal ions, ligand types and even different oxidation states can adjust this energy gap.
• For example, Copper(II) sulfate solution is blue because it absorbs red and orange light, the complementary color of which is blue.

Effect of Ligands on Colour of Complexes

• The type of ligand attached to the transition metal can significantly change the colour of a complex. This is because the different ligands can cause the d orbitals to split into two different energy levels.
• Ligands that cause a large energy gap create a complex that absorbs lower wavelength (more energetic) light, creating a colour at the opposite end of the colour spectrum.
• Strong field ligands like CN− and CO cause a large splitting, hence complexes with these ligands often appear blue/violet.
• Weak field ligands like I− and Br− cause a smaller splitting, resulting in complexes that appear red/orange.

Uses of Coloured Complexes

• Coloured complexes have various applications. For instance, they are used in the creation of coloured compounds in paints and inks, electronic devices, and photochemical applications.
• Some coloured complexes are used as catalysts in a variety of industrial processes.

In summary, the richness in colour of the transition metal complexes is primarily due to the splitting of the d orbitals in these metals when they bind to ligands, leading to various energy levels and thus various colours.