Crystal field splitting energy (∆)∝ charge or oxidation state of central metal atom. Crystal field splitting energy (∆)∝ Field strength of ligand (and chelation) Crystal field stabilisation energy (CFSE) =[−0.4t2g. +0.6 eg] ∆∘ (for octahedral)
For, [Co(en)3]3+:Co3+:t2g6eg0;CFSE=−2.4(∆0)1 [Co(NH3)6]3+:Co3+:t2g6eg0;CFSE=−2.4(∆0)2 [Co(NH3)6]2+:Co2+:t2g5eg2;CFSE=−0.8(∆0)3 [Co(NH3)4]2+:Co2+:e4t23;CFSE=−1.2∆t ( as :(∆t)=