Explain why transition metal ions are usually coloured.

Explanation

Transition metal ions are usually coloured because of the absorption of radiation from visible light region 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 observed corresponds to the complementray colour of the light absorbed . The frequency of light absorbed is determined by the nature of ligand .
When ligands has free electrons , the transfer of electron from anion to cation takes palce . In this case , energy is absorbed during this process which is responsible for colour .
Thus , most of the compounds fo transition metals are coloured due to d-d transition . Thransition metal ions having `d^(0)` configuration are colourless .