The electronic configuration of Mn2+ is 3d5 , which is stable due to half-filled sub-shell. Hence, Mn2+ is not easily oxidised to Mn3+ . On the other hand, Fe2+ has electronic configuration 3d6 . After losing one electron it changes to Fe3+ which has stable 3d electronic configuration. Hence, Fe2+ is relatively easily oxidised to Fe3+
Mn2+ \(\longrightarrow\) Mn3+ + e-
3d5(stable) 3d4
Fe2+ \(\longrightarrow\) Fe3+ + e-
3d6 3d5(stable)