Electronic configuration of
Mn2+ is
4s03d5 :
Electronic configuration of
Fe2+ is
4s03d6 :
Since, in
Mn2+ the d-orbital is completely half-filled, therefore, its third ionization energy is high and hence, it stable towards oxidation to +3 oxidation states, whereas, in
Fe2+, the d-orbital is partially filled and only one electron is paired, therefore, its third ionization energy is lower than
Mn2+ ion and hence, it is less stable towards +3 oxidation states.