The crystal field stabilization energy (CFSE) is the stability that results from placing a transition metal ion in the crystal field generated by a set of ligands. It arises due to the fact that when the d-orbitals are split in a ligand field (as described above), some of them become lower in energy than before with respect to a spherical field known as the barycenter in which all five d-orbitals are degenerate. For example, in an octahedral case, the t set becomes lower in energy than the orbitals in the barycenter. As a result of this, if there are any electrons occupying these orbitals, the metal ion is more stable in the ligand field relative to the barycenter by an amount known as the CFSE. Conversely, the e orbitals (in the octahedral case) are higher in energy than in the barycenter, so putting electrons in these reduces the amount of CFSE.
If the splitting of the d-orbitals in an octahedral field is Δ, the three t orbitals are stabilized relative to the barycenter by / Δ, and the e orbitals are destabilized by / Δ. As examples, consider the two d configurations shown further up the page. The low-spin (top) example has five electrons in the t orbitals, so the total CFSE is 5 x / Δ = 2Δ. In the high-spin (lower) example, the CFSE is (3 x / Δ) - (2 x / Δ) = 0 - in this case, the stabilization generated by the electrons in the lower orbitals is canceled out by the destabilizing effect of the electrons in the upper orbitals.