ligandidien

Ligandidien is a term used in coordination chemistry to denote ligands—atoms, ions, or molecules that donate electron density to a central metal atom or ion to form a coordination complex. Ligandidien bind through donor atoms such as nitrogen, oxygen, sulfur, or carbon, and can be neutral or charged. They can be monodentate, bidentate, or polydentate, and their denticity influences the stability of the complex via the chelate effect. The electronic and steric properties of ligandidien determine the geometry and spin state of the metal center; classical examples include ammonia, phosphines, halides, and carbon monoxide. Multidentate ligandidien, such as ethylenediamine or 1,10-phenanthroline, form five- or six-membered chelate rings.

Ligandidien can act as hard or soft donors, affecting ligand field strength and back-donation in organometallic complexes. In catalysis and bioinorganic chemistry, the choice of ligandidien modulates reactivity, selectivity, and stability of metal centers. The study of ligandidien interfaces with ligand design, metal–ligand bonding theories, and spectroscopy to characterize structures and electronic properties. Etymology traces ligandidien to ligare ('to bind') plus suffixes found in some languages to form the plural. See also coordination compound, chelate, ligand field theory, eta notation.