Why E° value for the Mn3+/Mn2+ couple is highly positive (+1.57 V) as compared to Cr3+/Cr2+?

 Q. Why E° value for the Mn³⁺/Mn²⁺ couple is highly positive (+1.57 V) as compared to Cr³⁺/Cr²⁺?

Ans.  Much larger third ionization energy of Mn(where change is d⁵ to d⁴) is mainly responsible for this. This also explains that +3 state of Mn is of little importance.