Ligands

Ligands are molecules or ions that donate at least one electron pair to a central atom or ion, typically a transition metal, to form a coordination complex. They are typically Lewis bases and can be neutral or anionic. A ligand may bind through one donor atom (monodentate), two (bidentate), or more (polydentate); ligands binding through multiple atoms that form rings with the metal are called chelating ligands. Ambidentate ligands can bind through different atoms, giving different binding modes; examples include cyanide (binding via C or N) and thiocyanate (binding via S or N).

Common classes include neutral ligands such as water, ammonia, carbon monoxide, and pyridine, and anionic ligands such as chloride, hydroxide, cyanide, and acetate. The donor atoms are usually electronegative atoms such as nitrogen, oxygen, sulfur, phosphorus, or halides. Ligand denticity and the overall denticity influence the geometry and coordination number of the metal center.

Chemists distinguish hard and soft ligands in HSAB terms; hard ligands tend to bind hard acceptors, soft ligands bind soft acceptors. Strong-field ligands like CN− and CO cause larger d-orbital splittings than weak-field ligands like I−, which affects electronic structure and spin state.

Chelation, where a polydentate ligand wraps around the metal, often increases complex stability (the chelate effect) and can affect reactivity. Stability is quantified by formation constants; ligand lability ranges from rapid substitution to slow, depending on the system.

Ligands play central roles in catalysis, bioinorganic chemistry, materials science, and sensing. Biological examples include ligands bound to heme iron that enable oxygen transport and redox chemistry; industrial examples include metal carbonyls and many metal complexes used as catalysts.