Stereoisomerism

Stereoisomers of coordination compounds are molecules that have the same chemical formula and the same connectivity of the ligands to the metal atom but differ in the arrangement of the ligand molecules around the central atom.

Based on the molecular structure, stereoisomers of coordination compounds are classified as geometric isomers or optical isomers.

Geometric isomers are stereoisomers that have completely different molecular geometries and exist with distinct physical and chemical properties.

cis–trans isomers are an example of geometric isomers. They occur in square-planar complexes with two sets of two identical ligands, or MA₂B₂, and in octahedral complexes with a set of four identical ligands and a pair of another ligand, or MA₄B₂.

For example, in the cis isomer of the square-planar complex diamminedichloroplatinum(II), known as cisplatin, the chloride ligands are adjacent to each other and on the same side of the molecule. In the trans isomer, transplatin, the chloride ligands are on the opposite sides of the molecule.

Octahedral complexes with two sets of three identical ligands, or MA₃B₃, can exhibit another type of geometric isomerism, known as fac–mer isomerism.

For example, in the fac isomer of triamminetrichlorocobalt(III), the three amine ligands and three chloride ligands are on opposite sides of the molecule and thus make up opposite faces of the octahedron. In the mer isomer, three of the same ligands form an arc around the central metal atom.

The other class of stereoisomers is defined as optical isomers. Like our hands, optical isomers are mirror images of each other and non-superposable. These isomers, also called enantiomers, are described as chiral and have very few differences in their physical and chemical properties.

Optical isomers are distinguished by their interaction with plane-polarized light. Polarized light has electric field vectors oscillating in a single plane, which is rotated by a certain amount when the polarized light passes through a solution of an enantiomer.

For example, one enantiomer of tris(ethylenediamine)cobalt(III) rotates the plane to the right and is called dextrorotatory, or the d optical isomer. The other enantiomer rotates the plane to the left and is called levorotatory, or the l optical isomer.