CFSE of high-spin d5−Mn2+ complex is 0.
Octahedral complex splitting takes place in two orbitals named as
eg(doublydegenerate)
t2g(triplydegenerate)
d5beingahighspincomplexhencetheelectronsentersintoegorbital
Hence the energy of
egorbitalis−0.4Δandt2gorbitalis+0.6Δ
Hencethe3electronsoft2gbecomes3×(−0.4)Δ=−1.2Δ
Forthe2electronsofegorbitalitbecomes2×(+0.6)=1.2Δ
Hencedifferenceinenergy=1.2Δ−1.2Δ=0