Question

How do d-d transitions impart colour to transition metal ions?

Answer 1

1. Transition metal ions have partially filled d-orbitals, which split into two energy levels (t_2g and e_g) in the presence of ligands (crystal field splitting).
2. When visible light hits these ions, electrons in lower energy d-orbitals absorb specific wavelengths of light and get excited to higher energy d-orbitals (d-d transitions).
3. The absorbed light’s wavelength is specific to the energy gap created by splitting, so only certain colours are absorbed.
4. The complex appears coloured because it transmits or reflects the complementary colour of the absorbed light.
5. Ions with completely full or empty d-orbitals are colorless, as no d-d transitions occur.