Ligante

Ligante, in the context of coordination chemistry, refers to a molecule or ion that binds to a central metal atom or ion to form a coordination complex. Ligands donate one or more pairs of electrons to the metal, acting as Lewis bases toward the metal center, which behaves as a Lewis acid. Binding occurs through coordinate covalent bonds, though the resulting complex is often described by classical bonding models.

Ligands are classified by denticity: monodentate ligands bind through a single donor atom (for example NH3, H2O, Cl−); polydentate or chelating ligands bind through multiple donor atoms (for example en, oxalate, EDTA). Chelating ligands form rings with the metal, often increasing complex stability due to the chelate effect. Ambidentate ligands can bind through more than one kind of donor atom (for example SCN− can bind via S or N).

Common donor atoms include nitrogen, oxygen, sulfur, and phosphorus. The coordination number, i.e., the number of donor atoms attached to the metal, varies with metal and ligand geometry; typical values are 4 (tetrahedral or square planar) and 6 (octahedral).

Ligands influence the properties of the complex, including geometry, color, magnetism, and reactivity. They are also categorized by their effect on the metal’s d-orbital energies, described by ligand field theory and the spectrochemical series.

In Portuguese and Spanish, ligante is the term for ligand in coordination chemistry. Applications span catalysis, bioinorganic chemistry (e.g., heme and chlorophyll), and metal chelation therapies.