The $E^{\circ }$ value for the $\mathrm{Mn}{}^{3+}$/$\mathrm{Mn}{}^{2+}$ couple is more positive than that of $\mathrm{Cr}{}^{3+}$/$\mathrm{Cr}{}^{2+}$ or $\mathrm{Fe}{}^{3+}$/$\mathrm{Fe}{}^{2+}$ due to the change of

Solution & Explanation

\textbf{Answer:} (b) $\mathrm{d}{}^4$ to $\mathrm{d}{}^5$ configuration \textbf{Solution:} The $\mathrm{Mn}{}^{3+}$/$\mathrm{Mn}{}^{2+}$ reduction is $\mathrm{Mn}{}^{3+}+\mathrm{e}{}^{-}\to \mathrm{Mn}{}^{2+}$, which converts $3\mathrm{d}{}^4$ ($\mathrm{Mn}{}^{3+}$) into $3\mathrm{d}{}^5$ ($\mathrm{Mn}{}^{2+}$). The product $\mathrm{Mn}{}^{2+}$ has the exceptionally stable half-filled $3\mathrm{d}{}^5$ configuration, so this reduction is strongly favoured. This extra stability of $\mathrm{Mn}{}^{2+}$ makes $\mathrm{Mn}{}^{3+}$ easy to reduce, giving a highly positive $E^{\circ }$ for the $\mathrm{Mn}{}^{3+}$/$\mathrm{Mn}{}^{2+}$ couple compared with the $\mathrm{Cr}{}^{3+}$/$\mathrm{Cr}{}^{2+}$ and $\mathrm{Fe}{}^{3+}$/$\mathrm{Fe}{}^{2+}$ couples.