Why is the `E^(Theta)` value for the `Mn^(3+)//Mn^(2+)` couple much postive than for `Cr^(3+)//Cr^(2+) or Fe^(3+)//Fe^(2+)`? Example

Explanation

`{:(Mn(Z=25):[Ar]^(18)4s^(2)3d^(5),Cr(Z=24):[Ar]^(18)4s^(1)3d^(5)),(Mn^(2+):[Ar]^(18)3d^(5),Cr^(2+):[Ar]^(18)3d^(4)),(Mn^(3+):[Ar]^(18)3d^(4),Cr^(3+):[Ar]^(18)3d^(3)):}`
The electronic configuration of `Mn^(2+)` ion is more symmetrical as compared to that of `Cr^(2+)` ion. Therefore, third ionisation potential of `Mn^(2+)` is much higher. Consequently `E^(@)` value of the `Mn^(3+)//Mn^(2+)` couple is much more positive than for `Cr^(3+)//Cr^(2+)` couple.