Why is `Cr^(2+)` reducing and `Mn^(3+)` oxidising when both have `d^(4)` configuration ?

Explanation

`Cr^(2+)` is reducing as its configuration changes from `d^(4)` to `d^(3)` , the latter having a half-filled `t_(2g)` level (see Unit 9) . On the other hand, the change from `Mn^(2+)` to `Mn^(3+)` results in the half-filled `(d^(5))` configuration which has extra stability.