Multidentate

Multidentate ligands, also called polydentate ligands, are molecules that can donate electrons to a central metal atom or ion from two or more donor sites. The number of donor atoms bound to the metal from a single ligand is its denticity. Ligands with two, three, four, or more donor sites are described as bidentate, tridentate, tetradentate, hexadentate, etc. Multidentate binding often forms chelate rings that wrap around the metal center, increasing complex stability.

Classic examples include ethylenediamine (en), a bidentate amine; oxalate, C2O4^2−, a bidentate; and EDTA (ethylenediaminetetraacetic acid), a hexadentate ligand that binds through four carboxylate oxygens and two amine nitrogens to form highly stable octahedral complexes with many metals. Macrocyclic ligands like DOTA can be octadentate, conferring high kinetic and thermodynamic stability. The chelate effect explains why multidentate ligands often produce more stable complexes than an equivalent number of monodentate ligands.

Ambidentate ligands can bind through different atoms but typically at one site; multidentate ligands bind through multiple donor atoms simultaneously. Multidentate ligands are central to advances in catalysis, magnetic resonance imaging contrast agents, and metal-based therapeutics, where stability and selectivity of metal complexes are important.