Question

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

• A. $MnO _{4}^{2-}$
• B. $CrO^{2-}_4 $
• C. $MnO^{-}_4 $
• D. $Cr_2 O^{2-}_7 $

Answer 1

The Correct Option is A

To determine which ion exhibits both $d-d$ transition and paramagnetism, we need to consider the electronic configuration and the oxidation state of the central metal ion in each given option.

1. $MnO_4^{2-}$: The manganese ion in $MnO_4^{2-}$ is in the +6 oxidation state. In this state, manganese has the electronic configuration [Ar] 3d¹ 4s⁰. The presence of one unpaired electron in the 3d orbital means the ion is paramagnetic. The $d-d$ transition is possible since the unpaired electron in the d orbital can undergo electronic transition.
2. $CrO_4^{2-}$: The chromium ion in $CrO_4^{2-}$ is in the +6 oxidation state, corresponding to a 3d⁰ configuration. Since there are no unpaired electrons, it is not paramagnetic, and $d-d$ transitions are not possible.
3. $MnO_4^{-}$: The manganese in $MnO_4^{-}$ is in the +7 oxidation state. Manganese in this state also has a 3d⁰ configuration, leading to no unpaired electrons and hence no paramagnetism or $d-d$ transitions.
4. $Cr_2O_7^{2-}$: The chromium in $Cr_2O_7^{2-}$ is in the +6 oxidation state, similar to $CrO_4^{2-}$ with a 3d⁰ configuration. It does not exhibit paramagnetism or $d-d$ transitions either.

After analyzing each option, $MnO_4^{2-}$ is the only ion that satisfies the conditions of having a $d-d$ transition due to the presence of an unpaired electron, as well as displaying paramagnetism.

Thus, the correct answer is $MnO_4^{2-}$.