Question

why is Cr3+ reducing and Mn3+ oxidising when both have d4 configuration?

Answer 1

both dont have d4 config
Mn has d4 Cr has d3
by gaining one electron Mn forms
d5 stable half filled config
therefore oxidising

Answer 2

Solution — Cr$^{2+}$ is reducing as its configuration changes from d$^{4}$ to d$^{3}$ , the latter having a half-filled t$_{2g}$ level.

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