Why is Cr2+ reducing and Mn3+ oxidising when both have d4 configuration?
Cr2+ has the configuration 3d4. It can lose electron to form Cr3+ which has the stable 3d3 configuration (as it has half-filled t2g level-explained in unit 9). Hence, it is reducing. On the other hand, Mn3+ also has 3d4 configuration but it can gain electron to form Mn2+ which has stable 3d5 configuration (as it is exactly half-filled). Hence, it is oxidizing.
Alternatively, Eº value for Cr3+/Cr2+ is negative (–0.41 V) whereas Eº value for Mn3+/Mn3+ is positive (+1.57 V). Hence, Cr2+ ion can easily undergo oxidation to give Cr3+ ion and, therefore, acts as strong reducing agent whereas Mn3+ can easily undergo reduction to give Mn2+ and hence acts as oxidizing agent.