Ligands split the d orbitals found in the transition metal ion in the complex. Different ligands split the d orbitals differently. A partially filled d orbital will absorb certain frequencies of light, leaving the complex itself to appear the complimentary colour of the light absorbed. Electron(s) promoted to a higher energy d orbital are responsible for the specific wavelengths of light absorbed.
![](https://i.ytimg.com/vi/9NzERGvNc5Y/maxresdefault.jpg)