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. On the other hand, the change from `Mn^(3+) " to " Mn^(2+)` results in the half-filled `(d^(5))` configuration which has extra stability.