optical isomerism of complexes

Hold up your two hands. They are mirror images of each other, yet no matter how you turn them, you cannot lay one exactly on top of the other — a left glove never fits the right hand. Some complexes are like that: they exist as two forms that are mirror images but not superimposable. This is optical isomerism, and the two forms are called enantiomers.

A complex is chiral (handed) when it has no internal mirror plane and no center of inversion — no internal symmetry that would let its mirror image be rotated back onto itself. The most common cause in coordination chemistry is chelation: an octahedral complex with three bidentate ligands, like [Co(en)3]3+, twists into a propeller that comes in a left-handed and a right-handed form, labeled lambda and delta. Cis isomers of certain MA2B2-type chelated complexes are also chiral, while the corresponding trans isomers (which have a mirror plane) are not. The name 'optical' comes from how we detect them: the two enantiomers are identical in nearly every ordinary property, but they rotate the plane of polarized light in equal and opposite directions, one clockwise (dextrorotatory) and one counterclockwise (levorotatory). It was exactly such an optical resolution of a chiral complex containing no carbon that let Werner prove his geometry was real, silencing critics who claimed only carbon could be the source of chirality.

Optical isomerism matters far beyond a curiosity: chirality governs how molecules interact with the handed environment of living things, so a metal complex's two enantiomers can have different biological effects, and chiral metal catalysts are used industrially to make one mirror-image form of a drug selectively. Importantly, a complex can also be chiral with no chelate rings at all if its overall arrangement lacks any mirror symmetry; chelation simply makes chirality common.