Crystal field theory (CFT) describes the breaking of orbital degeneracy in transition metal complexes due to the presence of ligands. CFT qualitatively describes the strength of the metal-ligand bonds. Based on the strength of the metal-ligand bonds, the energy of the system is altered. This may lead to a change in magnetic properties as well as color. This theory was developed by Hans Bethe and John Hasbrouck van Vleck.

According to CFT, the attraction between the central metal and ligands in a complex is purely electrostatic. 

The theory is developed by considering energy changes of the five degenerate d- orbitals being surrounded by an array of point charges consisting of the ligands. As a ligand approaches the metal ion, the electrons from the ligand will be closer to some of the d-orbitals and farther away from others, causing a loss of degeneracy. The electrons in the d-orbitals and those in the ligand repel each other due to repulsion between like charges. Thus, the d-electrons closer to the ligands will have a higher energy than those further away, which results in the d-orbitals splitting in energy.

This splitting is affected by the following factors: 

•    The nature of the ligands. The stronger the ligand, the greater is the splitting.
•    Oxidation state of the central metal ion. A higher oxidation state leads to larger splitting. 
•    Size of d orbitals (i.e., transition series).
•    Geometry of the complex.