Colour in Transition Metal Complexes

One of the most visually striking properties of transition metal chemistry is colour. Solutions of copper(II) sulfate are blue, potassium permanganate is deep purple, and chromium(III) compounds are green. This lesson explains why transition metal complexes are coloured and what factors influence the colour observed.

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d-Orbital Splitting

In an isolated transition metal ion, all five d orbitals have the same energy — they are said to be degenerate. However, when ligands surround the metal ion to form a complex, the ligands' lone pairs create an electric field that interacts differently with the different d orbitals.

Some d orbitals point directly at the ligands and experience greater repulsion, raising their energy. Others point between the ligands and experience less repulsion. This creates an energy gap (ΔE) between two sets of d orbitals — the d orbitals have been split.

In an octahedral complex:

• Two d orbitals (d(x²−y²) and d(z²)) are raised in energy — they point at the ligands
• Three d orbitals (d(xy), d(xz), d(yz)) are lowered in energy — they point between the ligands

The energy difference between these two sets is called the crystal field splitting energy (Δoct or ΔE).

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