Question

How would you account for the following:

Of the d⁴ species, Cr²⁺ is strongly reducing while manganese (III) is strongly oxidising.

Answer 1

1. Both Cr²⁺ and Mn³⁺ have d⁴ electronic configurations, but their contrasting behaviours arise from the stability of their resulting oxidation states. Cr²⁺ is strongly reducing because it tends to lose one electron to form Cr³⁺ (d³ configuration).
2. The d³ configuration has a half-filled t_2g subshell in an octahedral field, which is particularly stable due to symmetric electron distribution and lower energy.
3. On the other hand, Mn³⁺ is strongly oxidising because it tends to gain one electron to form Mn²⁺ (d⁵ configuration). The d⁵ configuration corresponds to a half-filled d-subshell, which is highly stable due to exchange energy and symmetry.
4. Thus, Cr²⁺ undergoes oxidation to achieve greater stability, while Mn³⁺ undergoes reduction for the same reason.

Answer 2

Due to the transition from 3d⁴ to 3d³, Cr²⁺ is strongly decreasing. A more stable arrangement is the 3d³ configuration \[\ce{(t^3_{2g})}\]. The oxidising property of Mn³⁺ causes a transition from 3d⁴ to 3d⁵, which is an extra stable configuration. Mn³⁺ is strongly oxidising because of this.