Stereoisomerism in transition element complexes

cis–trans isomerism arises when a complex can place two identical ligands either adjacent or opposite:

• Square-planar MA₂B₂ — e.g. Pt(NH₃)₂Cl₂: cis (the two Cl at 90°) or trans (the two Cl at 180°).
• Octahedral MA₄B₂ — e.g. [Co(NH₃)₄Cl₂]⁺: cis (the two Cl adjacent, 90°) or trans (the two Cl opposite, 180°).

The two forms have the same bonds but a different spatial arrangement, so they are stereoisomers (often with different properties).

A complex with all identical ligands (e.g. [Cr(H₂O)₆]³⁺) has every position equivalent, so it shows no cis–trans isomerism.

Optical isomerism occurs when a complex is non-superimposable on its mirror image (chiral), giving a pair of enantiomers that rotate plane-polarised light in opposite directions.

In transition-metal chemistry this typically needs bidentate ligands. For example, [Ni(en)₃]²⁺ (en = 1,2-diaminoethane) has three bidentate ligands wrapped around the octahedral metal like a propeller — there are left- and right-handed forms that cannot be superimposed.

A complex with only monodentate ligands rarely shows optical isomerism, so look for bidentate (or polydentate) ligands as the clue.

Cisplatin is cis-Pt(NH₃)₂Cl₂, a square-planar complex used to treat cancer. Inside a cell its two chloride ligands are replaced (ligand substitution), and the platinum then bonds to two nitrogen atoms on DNA, cross-linking it and preventing replication — so the cancer cell cannot divide.

Only the cis isomer works: its two leaving Cl ligands are adjacent, so the platinum can bridge two neighbouring DNA nitrogen atoms. In the trans isomer the Cl ligands are opposite, the wrong geometry to bind DNA, so it is inactive.

This is a striking example of how stereoisomerism determines biological function — and why drug stereochemistry is so important.