Explain why transition metal ions are usually coloured.

Explanation

Transition metal ions are usually coloured because of the absorption of radiation from visible region of light to promote an electron from a lower energy d-orbital to a higher energy d-orbital . The energy of excitation corresponds to the frequency of light absorbed. The colour of the light absorbed. The colour observed corresponds to the complementary colour of the light absorbed
The frequency of light absorbed is determined by the nature of ligand.
When ligands have free electrons, the transfer of electron from anion to cation takes place. In this case, energy is absorbed during this process which is responsible for colour .
Thus, most of the compounds of transition metals are coloured due to d-d transition. Transition metal ions havieng `d^(0)` configuration are colourless.