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
Cr2+ is strongly reducing in nature. It has a d4 configuration. While acting as a reducing agent, it gets oxidised to Cr3+ (electronic configuration d3). This d3 configuration can be written as t32 configuration, which is a more stable configuration.
In the case of Mn3+(d4), it gains one electron to become Mn2+(d5). This has an exactly half filled d− orbital and is highly stable.