Which one of the following complexes shows optical isomerism?

The correct answer is Option (2) → cis\([Co(en)_2Cl_2]Cl\)

Optical isomers are mirror images that cannot be superimposed on one another; a complex shows optical isomerism only when it has no plane of symmetry. Optical isomerism is common in octahedral complexes with didentate ligands such as en.

In cis-[Co(en)₂Cl₂]⁺ the two Cl atoms are next to each other and the two en rings are arranged like the blades of a propeller. The complex has no plane of symmetry, so it exists as a d and an l form.

Why the other options are incorrect

• Option (1) \([Co(NH_3)_3Cl_3]\) — Incorrect. An MA₃B₃ complex; it shows facial and meridional geometrical isomers, but each has a plane of symmetry, so neither is optically active.

• Option (3) trans\([Co(en)_2Cl_2]Cl\) — Incorrect. The two Cl atoms are opposite each other; the plane containing the Co and the two en rings is a plane of symmetry, so the trans form is optically inactive.

• Option (4) \([Co(NH_3)_4Cl_2]Cl\) — Incorrect. An MA₄B₂ complex with only monodentate ligands; it shows cis and trans forms, both with planes of symmetry.

NCERT (2026-27 edition):
Optical Isomerism – “… Optical isomerism is common in octahedral complexes involving didentate ligands (Fig. 5.6). In a coordination entity of the type [PtCl₂(en)₂]²⁺, only the cis-isomer shows optical activity (Fig. 5.7).”