A reducing agent, or reductant, loses electrons and is oxidized in a chemical reaction. A reducing agent is typically in one of its lower possible oxidation states, and is known as the electron donor.
Cr2+ is reducing as it configuration changes from d4 to d3 due to formation of Cr3+, which has half filled t2g level, on other hand, the change Mn3+ to Mn2+ result half filled d5configuration which has extra stability.