Which one of the following ions exhibits d-d transition and paramagnetism as well?

Solution & Explanation

\textbf{Answer:} (d) $\mathrm{MnO}{}_4{}^{2-}$. \textbf{Solution:} For a d-d transition and paramagnetism, the central metal must have at least one d electron (a $\mathrm{d}{}^1$ or higher, partly filled d). $\mathrm{MnO}{}_4{}^{-}$ (Mn is $+7$), $\mathrm{Cr}{}_2\mathrm{O}{}_7{}^{2-}$ and $\mathrm{CrO}{}_4{}^{2-}$ (Cr is $+6$) all have a $\mathrm{d}{}^0$ metal centre. Their colour arises from ligand-to-metal charge transfer, and they are diamagnetic — no d-d transition. $\mathrm{MnO}{}_4{}^{2-}$ (manganate): Mn is in the $+6$ state, i.e. $\mathrm{d}{}^1$. With one unpaired d electron it is paramagnetic and can undergo a genuine d-d transition. Hence option (d) is correct.