lowcoordinate

Low-coordinate is a term used in coordination chemistry and related fields to describe metal centers or atoms with a coordination number that is unusually small for the element and ligand set involved. The coordination number (CN) counts the ligands bound to the central atom, including anionic and neutral ligands as well as bridging connections. Low-coordinate species are typically two- or three-coordinate, though coordinations as low as one can occur in highly unusual or highly reactive systems. The exact geometry depends on electronic configuration, ligand size, and metal center.

Two- and three-coordinate complexes are common among transition metals, main-group elements, and coinage metals. Linear two-coordinate complexes, such as some Au(I), Ag(I), and Cu(I) species with strong σ-donor ligands, are classic examples. Three-coordinate species can adopt trigonal planar or T-shaped geometries depending on the metal and ligands. Low-coordinate centers often arise when bulky ligands prevent aggregation or when electronic factors favor undercoordination.

Stabilization of low-coordinate centers is typically achieved with bulky or strongly donating ligands, such as bulky phosphines, N-heterocyclic carbenes, or β-diketiminate ligands, which shield the metal and suppress oligomerization. These species often exhibit high reactivity toward small molecules (hydrogen, carbon monoxide, nitrogen) and can display unusual oxidation states or spin states. In catalysis, low-coordinate intermediates can be key reactive steps in homogeneous catalytic cycles, and in materials science they are relevant at surfaces, edges, or defects where undercoordinated sites contribute to catalytic activity.

See also coordination chemistry, coordination number, agostic interaction, and surface catalysis.