Complex ions containing transition metals are usually coloured, whereas the similar ions from non-transition metals are not coloured. The reason is that the partly filled d orbitals must be involved in generating the colour in some way. Most of the complex of transition elements are coloured. This is due to the absorption of radiation from visible light region to excite the electrons from its one position to another position in d-orbitals. In the presence of ligands, d-orbitals split into two sets of different orbital energies. Here transition of electron takes place and emit radiation which falls on the visible light region.