linkageisomerism

Linkage isomerism is a type of structural isomerism observed in coordination chemistry where an ambidentate ligand can attach to a central metal atom or ion through different atoms or sites. In such compounds, two or more connectivity patterns exist for the same formula, differing in which atom of the ligand is bonded to the metal. Unlike geometrical or optical isomerism, the different isomers are distinguished by the linkage of the ligand, not the overall arrangement around the metal.

Common ambidentate ligands include nitrite (NO2-), which can bind through nitrogen (M–NO2) or through oxygen (M–ONO); thiocyanate (SCN-), which can bind through sulfur (M–SCN) or nitrogen (M–NCS); cyanate (OCN-), which can bind through oxygen (M–OCN) or through nitrogen (M–NCO); and azide (N3-), which can bind through different terminal nitrogens. The two forms are often labeled to reflect the linkage: nitro- vs nitrito-, thiocyanato- vs isothiocyanato-, cyanato- vs isocyanato-, azido- forms, depending on the ligand and system.

Interconversion can occur thermally or photochemically in some systems, while in others the linkage isomer is kinetically or thermodynamically favored depending on metal, oxidation state, and counterions. Spectroscopic methods, especially infrared and Raman, as well as X-ray crystallography, are used to identify the binding mode, as the different donor atoms produce characteristic M–N, M–O, M–S stretches.

See also: Ambidentate ligand, coordination chemistry, isomerism.