Optical Isomerism

Optical isomerism occurs when a molecule lacks any improper symmetry element, so that it and its mirror image cannot be superimposed; such chiral forms are called enantiomers and rotate plane-polarised light in opposite directions. In [Co(en)3]^{3+} three bidentate ethylenediamine ligands wrap around the cobalt as a three-bladed propeller, producing left-handed and right-handed twists, conventionally labelled lambda and delta. These two arrangements are non-superimposable mirror images, making the ion optically active. The presence of a plane of symmetry would forbid chirality, so that option is self-contradictory. Ethylenediamine is not ambidentate; it always binds through its two nitrogen atoms, so that option is irrelevant. A linear arrangement of chelate rings is geometrically impossible for an octahedral tris-chelate. This propeller chirality is the classic NCERT example of optical isomerism in coordination chemistry. Plausibility check: tris-chelate octahedral complexes are textbook chiral species, and the resolved enantiomers of [Co(en)3]^{3+} indeed show equal and opposite optical rotations, confirming the analysis.