LIGAND FIELD THEORY
The five d orbitals with distinct values of the magnetic quantum number (m) have the similar energy in a free atom or ion. In any compound they interact differently with the surrounding ligands and a ligand field splitting is performed. The common coordination is octahedral (Oh point group) with six surrounding ligands. Then two of the d orbitals are required at higher energy than the other three (dxz, dxy and dyz, known as t2g). Such a splitting happens in any transition metal compound with octahedral coordination, including aqua ions and several solids. Electronic transitions between eg and t2g orbitals show rise to colors, which are a similar feature of transition metal complexes, and allow Δo to be measured experimentally.
Although originally explained in terms of electrostatic repulsion between the ligands d and electrons, it is now defined that ligand field splittings come from the similar type of orbital overlap effects as donor-acceptor interactions