Which one of the following is expected to exhibit optical isomerism? $(\mathrm{en}=$ ethylenediamine)

• A

  cis- $\left[\mathrm{Pt}\left(\mathrm{NH}_3\right)_2 \mathrm{Cl}_2\right]$
• B

  trans-[ $\left[\mathrm{Pt}\left(\mathrm{NH}_3\right)_2 \mathrm{Cl}_2\right]$
• C

  cis- $\left[\mathrm{Co}(e n)_2 \mathrm{Cl}_2\right]^{+}$
• Correct answer
  D

  trans-[ $\left[\mathrm{Co}(e n)_2 \mathrm{Cl}_2\right]^{+}$

Solution

Trans $\left[\mathrm{Co}(e n)_2 \mathrm{Cl}_2\right]^{+}$is able to exhibit the phenomenon of optical isomerism because it can form a superimposable mirror image.
Related Theory The foremost criteria to exhibit the optical isomerism is that the molecule should be dissymmetric i.e. the molecule should not be superimposed on its mirror image. The dissymmetry results from the presence of a chiral carbon atom.

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