Question

Why is the trans-isomer of [Co(en)₂Cl₂]⁺ not optically active?

Answer 1

1. The trans-isomer of [Co(en)₂Cl₂]⁺ is not optically active because it has a plane of symmetry that makes it achiral.
2. In the trans-isomer, the two chloride ligands are positioned opposite each other (180° apart), and the two ethylenediamine (en) ligands also occupy positions that create a symmetrical, mirror-image-like structure.
3. As a result, the complex is superimposable on its mirror image and does not exist as a pair of enantiomers.
4. Optical activity arises only in the presence of chirality, which is characterised by a lack of symmetry.
5. The trans-isomer, being symmetric and achiral, does not rotate plane-polarized light and is therefore optically inactive.
6. In contrast, the cis-isomer has the two chloride ligands next to each other, creating an uneven, chiral complex.
7. the cis-isomer has the two different mirror images (Δ and Λ forms), which makes it optically active.