Question

Why is \( Cr^{2+} \) strongly reducing while \( Mn^{3+} \) is strongly oxidizing?

Hint:

Elements with an electronic configuration that leads to an unstable oxidation state (such as \( Cr^{2+} \) or \( Mn^{3+} \)) are more likely to undergo reduction or oxidation to attain more stable states.

Answer 1

The objective is to elucidate the strong reducing nature of \( Cr^{2+} \) and the strong oxidizing nature of \( Mn^{3+} \).

1. \( Cr^{2+} \) as a Reducing Agent:
A species acts as a reducing agent by readily losing electrons. \( Cr^{2+} \) has an electronic configuration of \( [Ar] 3d^4 \). Upon oxidation to \( Cr^{3+} \), it achieves the configuration \( [Ar] 3d^3 \). In octahedral complexes, this \( 3d^3 \) configuration corresponds to a half-filled \( t_{2g}^3 \) subshell, which confers significant stability due to exchange energy and symmetry. The standard reduction potential, \( E^0 (Cr^{3+}/Cr^{2+}) = -0.41 \, \text{V} \), is negative, signifying that the oxidation \( Cr^{2+} \rightarrow Cr^{3+} + e^- \) is thermodynamically favorable, thus establishing \( Cr^{2+} \) as a potent reducing agent.

2. \( Mn^{3+} \) as an Oxidizing Agent:
A species acts as an oxidizing agent by readily gaining electrons. \( Mn^{3+} \) possesses the electronic configuration \( [Ar] 3d^4 \). Reduction to \( Mn^{2+} \) results in the configuration \( [Ar] 3d^5 \), a half-filled subshell, which is exceptionally stable. The standard reduction potential, \( E^0 (Mn^{3+}/Mn^{2+}) = +1.51 \, \text{V} \), is highly positive, indicating that the reduction \( Mn^{3+} + e^- \rightarrow Mn^{2+} \) is energetically favorable, thereby classifying \( Mn^{3+} \) as a powerful oxidizing agent.

Conclusion:
\( Cr^{2+} \) exhibits strong reducing properties due to its oxidation to the stable \( Cr^{3+} \) ion (\( 3d^3 \), half-filled \( t_{2g} \)), evidenced by a negative reduction potential (\( E^0 = -0.41 \, \text{V} \)). Conversely, \( Mn^{3+} \) is a strong oxidizing agent because it is readily reduced to the stable \( Mn^{2+} \) ion (\( 3d^5 \), half-filled), characterized by a substantial positive reduction potential (\( E^0 = +1.51 \, \text{V} \)).