tetracoordinated

Tetracoordinated is an adjective used in chemistry to describe an atom, ion, or center of a molecule that is bonded to four ligands or substituents. The term is synonymous with four-coordinate and is commonly used when discussing coordination chemistry, organometallics, and organosilicon or organoboron compounds. The coordination number in tetracoordinated species is four.

Two geometries are most characteristic for four-coordinate centers: tetrahedral and square planar. The tetrahedral arrangement is typical for many main-group elements and for many four-coordinate metal complexes with soft ligands, such as carbon in methane or silicon in silicon tetrachloride, and for metal carbonyls like Ni(CO)4. The square-planar geometry is common for certain d8 transition metal centers (for example some nickel, palladium, or platinum complexes with strong-field ligands).

For carbon, tetracoordination corresponds to sp3 hybridization and a roughly tetrahedral bond framework. In boron chemistry, tetracoordinate boron occurs in borate-like species and boron compounds with four substituents, such as the tetrafluoroborate anion. In coordination chemistry, tetracoordinated metal centers contrast with higher coordination numbers in octahedral or higher complexes, and are often associated with distinctive reactivity and fluxionality depending on ligand set and electronic structure.