This is because, three electrons enter $t_{2 g}$ level and are unpaired. When fourth
electron is present its position (in $t_{2 g}$ or $-g$ level) will be determined by the
strength of the ligand field.
(જ્યાં, $AA-$ સમમિતિય દ્વિદંતીય લિગાન્ડ અને $a,b,c,d,e-$ એકદંતીય લિગાન્ડ)
$\left( {{C_2}O_4^{2 - } = } \right.$ ઓક્ઝલેટો $)$
[પ.ક્ર.: $Cr= 24,\,Mn= 25, \,Fe= 26, \,Co= 27$]