Question

Which one of the following ions exhibits d-d transition and paramagnetism as well?
(A) ${\text{MnO}}_4^ -$
(B) ${\text{CrO}}_4^{2 - }$
(C) ${\text{MnO}}_4^{2 - }$
(D) ${\text{C}}{{\text{r}}_2}{\text{O}}_7^{2 - }$

Answer 1

When one or more unpaired electrons are present, the ion is paramagnetic in nature. If all the electrons are paired, the ion is diamagnetic in nature. If no electrons are present in the d subshell or if all electrons are present, then d-d transition is not possible.

Complete step by step answer:
The electronic configuration of manganese atom is $\left[ {{\text{Ar}}} \right]{\text{ 3}}{{\text{d}}^5}{\text{4}}{{\text{s}}^2}$. The oxidation state of manganese in ${\text{MnO}}_4^ -$ ion is +7. The electronic configuration of Mn(VII) is $\left[ {{\text{Ar}}} \right]{\text{ 3}}{{\text{d}}^0}{\text{4}}{{\text{s}}^0}$.
In ${\text{MnO}}_4^ -$ ion, the manganese atom has zero unpaired electrons. Due to this, ${\text{MnO}}_4^ -$ is diamagnetic in nature. In ${\text{MnO}}_4^ -$ ion, d-d transitions are not possible as no electron is present in either the lower ${{\text{t}}_{{\text{2g}}}}$ level or the upper ${{\text{e}}_{\text{g}}}$ level.
Hence, the option (A) is the incorrect option.
The electronic configuration of chromium atom is $\left[ {{\text{Ar}}} \right]{\text{ 3}}{{\text{d}}^5}{\text{4}}{{\text{s}}^1}$. The oxidation state of chromium in ${\text{CrO}}_4^{2 - }$ ion is +6. The electronic configuration of Cr(VI) is $\left[ {{\text{Ar}}} \right]{\text{ 3}}{{\text{d}}^0}{\text{4}}{{\text{s}}^0}$ .
In ${\text{CrO}}_4^{2 - }$ ion, the chromium atom has zero unpaired electrons. Due to this, ${\text{CrO}}_4^{2 - }$ is diamagnetic in nature. In ${\text{CrO}}_4^{2 - }$ ion, d-d transitions are not possible as no electron is present in either the lower ${{\text{t}}_{{\text{2g}}}}$ level or the upper ${{\text{e}}_{\text{g}}}$ level.
Hence, the option (B) is the incorrect option.
The electronic configuration of manganese atom is $\left[ {{\text{Ar}}} \right]{\text{ 3}}{{\text{d}}^5}{\text{4}}{{\text{s}}^2}$. The oxidation state of manganese in ${\text{MnO}}_4^{2 - }$ ion is +6. The electronic configuration of Mn(VI) is $\left[ {{\text{Ar}}} \right]{\text{ 3}}{{\text{d}}^1}{\text{4}}{{\text{s}}^0}$ .
In ${\text{MnO}}_4^{2 - }$ ion, the manganese atom has one unpaired electron. Due to this, ${\text{MnO}}_4^{2 - }$ is paramagnetic in nature. In ${\text{MnO}}_4^{2 - }$ ion, d-d transitions are possible as the unpaired electron in the lower ${{\text{t}}_{{\text{2g}}}}$ level can be easily excited to upper ${{\text{e}}_{\text{g}}}$ level.
Hence, the option (C) is the correct option.
The electronic configuration of chromium atom is $\left[ {{\text{Ar}}} \right]{\text{ 3}}{{\text{d}}^5}{\text{4}}{{\text{s}}^1}$. The oxidation state of chromium in ${\text{C}}{{\text{r}}_2}{\text{O}}_7^{2 - }$ ion is +6. The electronic configuration of Cr(VI) is $\left[ {{\text{Ar}}} \right]{\text{ 3}}{{\text{d}}^0}{\text{4}}{{\text{s}}^0}$ .
In ${\text{C}}{{\text{r}}_2}{\text{O}}_7^{2 - }$ ion, the chromium atom has zero unpaired electron. Due to this, ${\text{C}}{{\text{r}}_2}{\text{O}}_7^{2 - }$ is diamagnetic in nature. In ${\text{C}}{{\text{r}}_2}{\text{O}}_7^{2 - }$ ion, d-d transitions are not possible as no electron is present in either the lower ${{\text{t}}_{{\text{2g}}}}$ level or the upper ${{\text{e}}_{\text{g}}}$ level. Thus option (D) is also incorrect.

Hence, the option (C) is the incorrect option.

Note: Due to d-d transition, the transition metal ions such as ${\text{MnO}}_4^{2 - }$ appear coloured. In some ions such as ${\text{MnO}}_4^ -$ the colour is due to charge transfer as d-d transitions are not possible.