Q.What is crystal field stabilization energy (CFSE)?

Step 1. Once the d orbitals split into t2g and eg (or t2 and e) sets, the electrons distribute themselves among these split orbitals according to Hund's rule and the relative sizes of Δ and the pairing energy P.

Step 2. The crystal field stabilisation energy (CFSE) quantifies how much LOWER in energy this actual, split-orbital electron arrangement is compared to a hypothetical reference state where the same electrons are spread evenly across all five d orbitals at one unsplit (isotropic/barycentre) energy.

Step 3. Formally, CFSE = {ELF} - {Eiso} = {[nt2g(-0.4) + neg(0.6)]Δ₀ + npP} - {n'pP}, where nt2g and neg are the electron counts in each split set, np is the number of electron pairs actually present in the ligand field, and n'p is the number of electron pairs present in the isotropic reference field. …

State CFSE as a DIFFERENCE (ligand-field energy minus isotropic-field energy), not just 'the energy of the split orbitals', and …

• Defining CFSE as simply Δ₀ itself, rather than the properly weighted combination of electron counts in each split set (plus any pairing-energy penalty) relative to the isotropic reference. …