bidentate

Bidentate ligands are ligands that bind to a central metal atom or ion through two donor atoms, forming a chelate ring. The two donor atoms are typically heteroatoms such as nitrogen, oxygen, or sulfur, but can be carbon-based in some cases. Because the two attachment points come from a single ligand, bidentate ligands often produce more stable metal complexes than comparable monodentate ligands, a phenomenon known as the chelate effect. The geometry of binding is influenced by the bite angle—the angle at which the donor atoms simultaneously engage the metal center—which can affect the overall geometry and reactivity of the complex. Bidentate ligands can be classified as chelating, if they form a closed chelate ring, or bridging, if they connect two metal centers.

Common examples include ethylenediamine (en), which binds through two amine nitrogens; 2,2'-bipyridine (bpy) and 1,10-phenanthroline (phen), which also bind through two nitrogens; and dianionic donors such as oxalate and malonate, which bind through two oxygen atoms. In many complexes, bidentate ligands enforce specific geometries such as octahedral or square-planar arrangements and are widely used in catalysis, materials chemistry, and bioinorganic models.

Compared with higher-dentate ligands, such as tridentate and tetradentate ones, bidentate ligands provide a simpler coordination mode and are frequently used to tune properties without excessively constraining the metal's coordination sphere.