COMEDK 2025 Afternoon Solved Paper

Concept:Optical activity occurs only when a coordination complex is chiral (lacks an improper axis of rotation).Explanation:Bidentate ethylenediamine (en) ligands form helical chelate rings in octahedral complexes.In $cis\text{-}[\mathrm{CoCl}_2(\mathrm{en})_2]^+$, the two chloride ligands are adjacent, breaking all planes of symmetry, so the complex exists as non‑superimposable $\Delta$ and $\Lambda$ enantiomers and is optically active.Similarly, $[\mathrm{Co}(\mathrm{en})_3]^{3+}$ and $cis\text{-}[\mathrm{PtCl}_2(\mathrm{en})_2]^{2+}$ are chiral due to the screw arrangement of the en ligands.In $trans\text{-}[\mathrm{CoCl}_2(\mathrm{en})_2]^+$, the two chloride ligands are opposite each other ($180^\circ$ apart), creating a plane of symmetry that passes through the cobalt ion and bisects the N–Co–N angles.This plane makes the complex achiral; its mirror image is superimposable, so it does not exhibit optical activity.Answer:Option C: $trans\text{-}[\mathrm{CoCl}_2(\mathrm{en})_2]^+$