tetracoordinate

Tetracoordinate describes an atom that forms four bonds to substituents or ligands, giving a coordination number of four. The term is used across inorganic, organometallic, and organic chemistry to describe species in which the central atom is bonded to four atoms or groups, and it can refer to either covalent or coordinate bonding depending on the bonding description.

Geometries of tetracoordinate centers vary by element and ligand environment. For many main-group elements, the most common arrangement is tetrahedral, as seen in methane (CH4) or silicon tetrachloride (SiCl4). In transition-metal chemistry, four ligands can also arrange in a square planar geometry, typical of d8 metal centers such as platinum(II) or nickel(II) complexes (for example, [PtCl4]2− or [Ni(CN)4]2−). Distortions from ideal geometries can occur due to ligand size, electronic effects, or chelation, leading to near-tetrahedral or other shapes.

Examples of tetracoordinate species include CH4 and SiCl4, as well as many metal complexes where the metal center is bonded to four ligands. In organic chemistry, a carbon atom that is tetracoordinate—bonded to four different substituents—is typically a stereocenter, provided there is no symmetry that makes two substituents equivalent.

In summary, tetracoordinate denotes four bonds to a central atom, encompassing a range of geometries and contexts from simple covalent compounds to complex coordination chemistry. See also coordination number, tetrahedral geometry, and square planar geometry.