Question

Of the d⁴ species, Cr²⁺ is strongly reducing, while Mn³⁺ is strongly oxidising. Why?

Answer 1

A reducing agent is a substance that undergoes oxidation, that is, it loses electrons. Cr²⁺ is a strong reducing agent because it has a 3d⁴ electronic configuration and readily loses an electron to form a 3d³ configuration, which is more stable due to half-filled t_2g​ orbitals.

In contrast, Mn³⁺ has a 3d⁴ configuration and tends to gain an electron to form Mn²⁺ (3d⁵). This configuration is especially stable, so Mn³⁺ acts as a strong oxidising agent.