Why is the `E^(0)` value for the `Mn^(3+)|Mn^(2+)` couple much more positive than that for `Cr^(3+)|Cr^(2+)` or `Fe^(3+)|Fe^(2+)` ? Explain.

Explanation

Let us consider the 3rd ionisation process of Cr, Mn and `Fe : Cr^(2+)(3d^(4))to Cr^(3+)` (`3d^(3)` , half - filled `t_(2g)`level), `Mn^(2+)` (`3d^(5)` , half - filled d - subshell) `to Mn^(3+)(3d^(4))Fe^(2+)(3d^(6))toFe^(3+)` (`3d^(5)` , half - filled d - subshell) The removal of an electron the half - filled 3d - subshell of `Mn^(2+)` requires large amount of energy i.e., Mn has very large 3rd ionisation enthalpy. This explains why `E^(0)` value for `Mn^(3+)|Mn^(2+)` couple is much more positive than that for `Cr^(3+)|Cr^(2+)` or `Fe^(3+)|Fe^(2+)` coples. This also shows that + 3 state of Mn has little importance.