Why does the trans-isomer of the octahedral coordination complex [Co(en)2Cl2]+ exhibit zero optical activity, whereas the cis-isomer exists as a pair of optically active enantiomers?

Answer and explanation

Correct answer: C. The trans-isomer has an inversion center making it achiral, while the cis-isomer is non-superimposable

  1. Step 1: Optical activity in coordination compounds requires molecular chirality, the absence of an improper rotation axis (), particularly a plane of symmetry () or center of inversion ().
  2. Step 2: In trans-, the two chloride ligands lie on opposite axial vertices (), creating a plane of symmetry bisecting the ethylenediamine chelate rings and a center of inversion, making it superimposable on its mirror image (achiral and optically inactive).
  3. Step 3: In cis-, the two chlorides are adjacent (), destroying all symmetry planes, producing non-superimposable d- and l-enantiomers.
Answer
The trans-isomer possesses a plane of symmetry and center of inversion, making it achiral, whereas the cis-isomer is non-superimposable on its mirror image
Why not the others: 'The trans-isomer has unpaired electrons...' is false because is a low-spin diamagnetic complex; 'The cis-isomer undergoes rapid planar racemization...' contradicts its stable optical resolution; 'The trans-isomer contains monodentate ligands that rotate...' is irrelevant because chirality depends on whole-molecule point group symmetry.

Practise more Coordination Compounds and d- and f-Block Elements.

Free questions with an explanation for every one, and full timed NEET UG mock tests.

All Coordination Compounds and d- and f-Block Elements questionsTake a mock test

More Coordination Compounds and d- and f-Block Elements questions