Question

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

• A. $Mn{O}_4^{-}$
• B. $Cr{O}_4^{2-}$
• C. $C{r}_2{O}_7^{2-}$
• D. $Mn{O}_4^{2-}$

Answer 1

Answer: (D)

In case of:

(i) ${Mn{O}_4}^{-}$, Mn is in the $+7$ oxidation state which means it has zero d $\left(d^o\right)$ electrons. Hence, there is no d-d transition and no paramagnetism. Its colour is due to charge transfer spectra.

(ii) ${Cr{O}_4}^{2-}$, Cr is in the $+6$ oxidation state which means it has zero d $\left(d^o\right)$ electrons. Hence, there is no d-d transition and no paramagnetism.

(iii) ${Mn{O}_4}^{2-}$, Mn is in the $+6$ oxidation state which means there is one d $\left(d^1\right)$ electron which gives rise to d-d transition and paramagnetism.

(iv) $C{r}_2{O}_7^{2-}$, Cr is in the $+6$ oxidation state which means it has zero d $\left(d^o\right)$ electrons. Hence, there is no d-d transition and no paramagnetism. Its colour is due to charge transfer spectra.

Hence, option C is the correct answer.