Most of transition metal ions exhibit characteristic colours in aqueous solutions.
The presence of unpaired d-electrons in transition metal ions in an aqueous solution gives them their colour. These unpaired d-electrons are stimulated to a higher energy d-orbital of the same n value from a lower energy level of a metal ion in a complex. The frequency of light absorbed corresponds to the energy of excitation, and this frequency is visible. The colour seen is the complementary colour of the light that has been absorbed. The type of ligand determines the frequency of light absorbed. Water molecules are ligands in aqueous solutions, and the colours of the ions detected are mentioned in the table below.
