CFSE = (-0.4 x $n_{t2g}$ + 0.6 x $n_{eg}$) $Delta_{oct}$. For low spin complexes, add pairing energy correction. Maximum CFSE in octahedral: d3 (high spin) and d6 (low spin) give -1.2 and -2.4 $Delta_{oct}$ respectively. Applications: (1) d3 Cr3+ and d6 Co3+ (low spin) form kinetically inert complexes. (2) Explains Irving-Williams series: Mn2+ < Fe2+ < Co2+ < Ni2+ < Cu2+ > Zn2+ for stability of divalent metal complexes. (3) Explains why spinel structures prefer certain cation distributions. (4) d0 and d5(HS) and d10 have zero CFSE — their complexes show no crystal field preference.