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

• A

  ${\mathrm{MnO}}_4^{-}$
• B

  ${\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}$
• C

  ${\mathrm{CrO}}_4^{2-}$
• Correct answer
  D

  ${\mathrm{MnO}}_4^{2-}$

Solution

$\begin{array}{ccc}{\mathrm{CrO}}_4^{-2}& {\mathrm{Cr}}^{+6}& \mathrm{diamagnetic}\end{array}$

$\begin{array}{ccc}{\mathrm{Cr}}_2{\mathrm{O}}_7^{-2}& {\mathrm{Cr}}^{+6}& \mathrm{diamagnetic}\end{array}$

$\begin{array}{ccc}{\mathrm{MnO}}_4^{-}& {\mathrm{Mn}}^{+7}& \mathrm{diamagnetic}\end{array}$

$\begin{array}{ccc}{\mathrm{MnO}}_4^{-2}& {\mathrm{Mn}}^{+6}& \mathrm{Paramagnetic}\end{array}$   

Unpaired electron is present so d-d transition is possible.

Hence, the correct answer is option: 'D'.