Q.a. What is optical isomerism? Draw the structure of optical isomers of [Cr(Ox)$_3$]$^{3-}$ OR b. What are chelating ligand? Give an example.

Optical isomers are non-superimposable mirror images of each other (chiral molecules); the octahedral tris-oxalato complex [Cr(C$_2$O$_4$)$_3$]$^{3-}$, with three symmetric bidentate ligands, exists as a pair of such mirror-image (d/l) optical isomers.

Optical isomerism occurs in complexes that are chiral, i.e. whose mirror image cannot be superimposed on the original structure. Such isomers, called enantiomers, are optically active — they rotate the plane of plane-polarised light in equal but opposite directions (dextro, d, and laevo, l, or equivalently labelled Δ and Λ).

[Cr(C$_2$O$_4$)$_3$]$^{3-}$: Chromium is octahedrally coordinated by three oxalate (C$_2$O$_4^{2-}$) ligands, each a symmetric bidentate (chelating) ligand. This tris-chelate arrangement, like [Co(en)$_3$]$^{3+}$, has no internal plane of symmetry — the whole octahedral "propeller" arrangement of the three chelate rings is chiral, and its mirror image is a distinct, non-superimposable structure. So [Cr(C$_2$O$_4$)$_3$]$^{3-}$ exists as a pair of optical isomers, mirror images of each other (commonly labelled Δ- and Λ-[Cr(ox)$_3$]$^{3-}$, or d- and l-).

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