Question

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

• A. \(\text{CrO}^{2-}_4\)
• B. \(\text{MnO}^{-}_4\)
• C. \(\text{Cr}_2\text{O}^{2-}_7\)
• D. \(\text{MnO}^{2-}_4\)

Answer 1

The Correct Option is D

To determine which ion exhibits a d-d transition and is paramagnetic, we need to examine the electronic configuration and oxidation states of the chromium and manganese ions in the options.

1. \( \mathrm{CrO_4^{2-}} \): In this ion, chromium is in the +6 oxidation state. Chromium in this oxidation state has the electronic configuration of \( [\mathrm{Ar}]\,3d^0 \), meaning there are no d-electrons to exhibit d-d transitions or contribute to paramagnetism. Thus, this ion does not exhibit d-d transition or paramagnetism.
2. \( \mathrm{MnO_4^-} \): Here, manganese is in the +7 oxidation state. The electronic configuration for manganese in this state is \( [\mathrm{Ar}]\,3d^0 \), so it lacks d-electrons, which means no d-d transition or paramagnetism.
3. \( \mathrm{Cr_2O_7^{2-}} \): In this ion, chromium is in the +6 oxidation state, with the electronic configuration of \( [\mathrm{Ar}]\,3d^0 \), hence no d-d transitions or paramagnetism occur, similar to \( \mathrm{CrO_4^{2-}} \).
4. \( \mathrm{MnO_4^{2-}} \): Manganese here is in the +6 oxidation state. The electronic configuration for manganese in this state is \( [\mathrm{Ar}]\,3d^1 \). This single d-electron is available for d-d transitions and also contributes to paramagnetism due to its unpaired nature.

Thus, the ion that exhibits both d-d transition and paramagnetism is \( \mathrm{MnO_4^{2-}} \), as it has an unpaired d-electron that allows for such characteristics.