Electronic configuration of Mn2+ is [Ar]183d5 and Electronic configuration of Fe2+ is [Ar]18 3d6 . It is known that half-filled and fully-filled orbitals are more stable. Therefore, Mn in (+2) state has a half-filled stable configuration, whereas the Fe in +3 oxidation state has partially filled subshells, which are relatively unstable. This is the reason Mn2+ shows resistance to oxidation to Mn3+. Also, Fe2+ has 3d6 configuration and by losing one electron, it attains half-filled stable configuration. Hence, Mn2+ compounds more stable than Fe2+ towards oxidation to their +3 state.
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