For questions relating to the electronic structure of inorganic complexes, particularly the splitting of degenerate $d$ or $f$ orbitals by the symmetry of the surrounding ligands.
Crystal field theory relates the splitting of degenerate $d$ or $f$ orbitals based on the electrostatic field of surrounding ligands. It describes some magnetic properties, colors, but not bonding, which requires a more complete molecular orbital or ligand field theory treatment.
According to CFT, the interaction between a transition metal and ligands arises from the attraction between the positively charged metal cation and negative charge on the non-bonding electrons of the ligand. 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.