Identify the optically active compounds from the following:
(A) \( [Co(en)_3]^{3+} \)
(B) \( trans-[Co(en)_2Cl_2]^+ \)
(C) \( cis-[Co(en)_2Cl_2]^+ \)
(D) \( [Cr(NH_3)_5Cl] \)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A) and (C) only
A complex is optically active when it is chiral: its mirror image cannot be superimposed on it, so it exists as a pair of enantiomers (d and l). A complex that has a plane of symmetry is identical to its mirror image and is optically inactive. Octahedral complexes with two or three didentate ligands (en = ethane-1,2-diamine) are the common chiral cases.
(A) \( \ce{[Co(en)3]^3+} \) — Correct. The three en rings are arranged like the blades of a propeller, which can turn either way; the complex has no plane of symmetry and exists as d and l forms.
(B) trans-\( \ce{[Co(en)2Cl2]+} \) — Incorrect. The two Cl atoms are opposite each other and both en rings lie in one plane, which is a plane of symmetry, so the mirror image is the same complex.
(C) cis-\( \ce{[Co(en)2Cl2]+} \) — Correct. With the two Cl atoms next to each other, the en rings are twisted out of one plane and no plane of symmetry is left; the cis isomer exists as a pair of enantiomers.
(D) \( \ce{[Cr(NH3)5Cl]} \) — Incorrect. A complex of the type \( \ce{MA5B} \) has several planes of symmetry passing through the Cr–Cl axis, so it is not chiral.