Monodentate

Monodentate refers to a ligand that coordinates to a metal center through a single donor atom, donating one pair of electrons to form one coordinate bond. The term distinguishes such ligands from dentate ligands that use two or more donor atoms. A monodentate ligand binds at one site on the metal and typically occupies one coordination position in the complex.

Common examples include water (aqua, H2O), ammonia (ammine, NH3), halide ions such as chloride (Cl−), hydroxide (OH−), fluoride (F−), carbon monoxide (CO) and neutral phosphines like triphenylphosphine (PPh3). These ligands differ in electronic properties and steric size, which influences the geometry and reactivity of the resulting complex. In most cases, a monodentate ligand binds through a single atom capable of donating electron density, such as oxygen, nitrogen, sulfur, phosphorus, or carbon.

Monodentate ligands are contrasted with bidentate, tridentate, and other polydentate ligands, which use two or more donor atoms to bind the metal, often forming chelate rings. The chelate effect explains why multi-dentate ligands can form more stable complexes than equivalent numbers of monodentate ligands. Nevertheless, monodentate ligands play a crucial role in many coordination compounds, including many well-known coordination complexes and catalysis systems, where their simple binding mode allows flexible geometries and facile ligand substitution.