Why do the transition metals componds are coloured ?

Explanation

Many transition metal ions are coloured. It is due to the presence of unpaired d-electrons. It can be explained on the basis of crystal field theory. When a ligand approaches the transition metal ion, their d-orbitals split into two degenerate sets. First set contains `d_(x^2 - y^2)` and `d_(x^2)` having higher energy. The other set containing `d_(xy)d_(yz), d_(zx)` have lower energy. Due to small energy difference, d-electrons can easily be excited from one set to another set. It is called d-d transition. So, when white light falls on ions, some of its energy corresponding to a certain colour is absorbed. The remaining colours of white light are transmitted and ions of transition metals appear coloured.