It is defined as the energy difference of electronic configurations in the ligand field (ELF) and the isotropic field / barycentre (Eiso)
CFSE (ΔE0) = {ELF} – {Eiso}
= {[nt2g – (0.4) + neg(0.6)] ∆0 +npP] – n’p P}
Where,
nt2g = the number of electrons in t,„ orbital.
neg = the number of electrons in n orbital
np = number of electron pairs in the ligand field.
n’p = number of electron pairs in the iso tropic field.