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How are diastereomers different from enantiomers in coordination chemistry?

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Structural diagram of the vitamin B12 coenzyme molecule, highlighting its organometallic core with five nitrogen-cobalt bonds and one cobalt-carbon bond at the center, surrounded by complex organic groups.
coordination compound Coordination compounds contain a central metal atom surrounded by nonmetal atoms or groups of atoms, called ligands. For example, vitamin B12 is made up of a central metallic cobalt ion bound to multiple nitrogen-containing ligands.

In coordination chemistry, the distinction between enantiomers and diastereomers lies in their spatial relationship and symmetry:

  • Enantiomers (or optical isomers) are molecules that are non-superimposable mirror images of each other. They possess the unique ability to rotate plane-polarized light in opposite directions. In coordination compounds, this occurs due to the presence of an asymmetric ligand or an asymmetric arrangement of ligands, such as in octahedral complexes containing three bidentate ligands like ethylenediamine.1

  • Diastereomers are isomers that are neither superimposable nor mirror images of one another. Unlike enantiomers, they do not share a mirror-image relationship. These isomers often arise in complexes with chelating ligands where the two ends of the ligand are chemically distinct, such as alanine or propylenenediamine, leading to multiple possible geometric arrangements around the metal center.1

Would you like to explore specific examples of geometric isomerism in coordination complexes?