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Question
How would you account for the following:
Of the d4 species, Cr2+ is strongly reducing while manganese (III) is strongly oxidising.
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Solution 1
- Both Cr2+ and Mn3+ have d4 electronic configurations, but their contrasting behaviors 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).
- The d3 configuration has a half-filled t2g subshell in an octahedral field, which is particularly stable due to symmetric electron distribution and lower energy.
- On the other hand, Mn3+ is strongly oxidizing 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.
- Thus, Cr2+ undergoes oxidation to achieve greater stability, while Mn3+ undergoes reduction for the same reason.
Solution 2
Due to the transition from 3d4 to 3d3, Cr2+ is significantly decreasing. A more stable arrangement is the 3d3 configuration \[\ce{(t^3_{2g})}\]. The oxidizing property of Mn3+ causes a transition from 3d4 to 3d5, which is an additional stable configuration. Mn3+ is severely oxidizing because of this.
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