Crystal Field theory

Many transition-metal complexes are coloured. This other spectroscopic properties, such as magnetism and hydration enthalpies, can be explained with the so-called crystal field theory (CFT). The CFT describes the degeneration of the d- and f-orbitals in transition-metal complexes. It does not attempt to describe any type of chemical bonds. CFT is based on the interaction of a positively charged cation and the nonbinding (negatively charged) electrons of the ligand. The general principle is that the five d orbitals are degenerated, meaning that they do not occupy the same energy level anymore. Once the ligands approach the central positively charged cation,
the electrons of the ligands will become closer to some of the d orbitals of the metal. This results in the degeneration of the d orbitals. Electrons in d orbitals that are closer to the ligands will occupy a higher energy level as the negatively charged electrons will repel each other.



Reference - Essential inorganic chemistry by katja
Page number 128