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:

(A), and (C) only

The correct answer is Option (3) → (A), and (C) only

(A) $[Co(en)_3​]^{3+}$ ($M(AA)_3$​ type)

• Structure: This complex has three bidentate ligands and possesses a helical structure.
• Symmetry: It does not have a plane of symmetry. It has $D_3$​ symmetry.
• Chirality: It exists as non-superimposable mirror images (Δ and Λ enantiomers) and was one of the first inorganic compounds to be resolved.
• Conclusion: Optically Active.

(B) $trans−[Co(en)_2​Cl_2​]^+$ ($trans−M(AA)_2​a_2$​ type)

• Structure: In the trans isomer, the two Cl ligands are opposite each other (at 180°).
• Symmetry: This structure possesses a plane of symmetry that bisects the Co−Cl bonds and passes through the two en ligands. It has $D_{2h}$​ point group symmetry.
• Chirality: It is an achiral molecule because it has a plane of symmetry, meaning the mirror image is superimposable.
• Conclusion: Optically Inactive.

(C) $cis−[Co(en)_2​Cl_2​]^+$ ($cis−M(AA)_2​a_2$​ type)

• Structure: In the cis isomer, the two Cl ligands are adjacent to each other (at 90°).
• Symmetry: This structure lacks a plane of symmetry. It has $C_2$​ symmetry.
• Chirality: It forms non-superimposable mirror images (enantiomers) and is chiral.
• Conclusion: Optically Active.

(D) $[Cr(NH_3​)_5​Cl]^{2+}$ ($Ma_5​b$ type)

• Structure: This complex has five identical monodentate ligands ($NH_3$​) and one different monodentate ligand (Cl).
• Symmetry: This structure always has a plane of symmetry that includes the Cr−Cl bond and two of the $Cr−NH_3$​ bonds.
• Chirality: Complexes of the $Ma_5​b$ type do not exhibit geometrical or optical isomerism, as they have an inherent plane of symmetry.
• Conclusion: Optically Inactive.

Final Selection

The optically active compounds are (A) $[Co(en)_3​]^{3+}$ and (C) $cis−[Co(en)_2​Cl_2​]^+$.