Stereoisomerism: Optical Isomerism

Optical isomers, or enantiomers, are pairs of molecules that are non-superimposable mirror images of one another — related to each other exactly the way a person's left and right hand are related. A molecule (or ion) shows this property, called chirality, whenever it lacks any internal plane of symmetry, centre of symmetry, or improper rotation axis; such a molecule is said to be chiral, or asymmetric.

Optical isomerism is common among octahedral complexes built from three identical bidentate chelating ligands, of the general type $\text{M}(\text{AA})_3$, because this arrangement has no plane that can reflect the whole structure onto itself. $[\text{Co}(\text{en})_3]^{3+}$ is the standard example: its three ethylenediamine ligands wind around the cobalt centre in a genuinely three-dimensional, propeller-like arrangement that has no internal mirror plane, making the whole ion chiral. It therefore exists as two distinct, non-superimposable mirror-image forms, conventionally labelled $\boldsymbol{d}$ (dextro) and $\boldsymbol{l}$ (laevo). Both forms are otherwise chemically identical — same melting point, same solubility, same colour, same reactivity toward achiral reagents — and differ physically in only one measurable way: each rotates the plane of plane-polarized light passing through it, but in opposite directions (one clockwise, the other anticlockwise, by an equal angle), which is precisely why this phenomenon is called optical isomerism. …