Fivecoordinate

Fivecoordinate refers to chemical species in which a central atom is bonded to five ligands, giving a coordination number of five. This arrangement is common in inorganic and organometallic chemistry, especially with transition metals, and can arise in various oxidation states and ligand environments.

The two most common geometries for five-coordinate complexes are trigonal bipyramidal and square pyramidal. In a trigonal bipyramidal geometry, three ligands occupy a plane in a roughly 120-degree arrangement, with two additional ligands occupying axial positions above and below the plane. In a square pyramidal geometry, four ligands form a square base around the central atom, with one ligand occupying the apex above the base. The geometry is influenced by the size, charge, and bite of the ligands, as well as electronic and steric factors of the metal center.

A notable feature of many five-coordinate species is fluxional behavior, particularly in trigonal bipyramidal systems. Berry pseudorotation can interchange axial and equatorial positions, making certain ligands appear equivalent in dynamic NMR experiments. The energy barrier for this process varies, so some five-coordinate complexes exchange rapidly at room temperature, while others are rigid and resolvable by spectroscopic methods.

Five-coordinate complexes are often reactive and serve as intermediates in catalytic cycles, such as oxidative addition, migratory insertion, or ligand association/dissociation steps. Classic examples include Fe(CO)5, a well-known trigonal bipyramidal complex, and numerous organometallic and coordination compounds with phosphine, amine, halide, or carbonyl ligands.

Characterization typically involves X-ray crystallography for solid-state geometry, and spectroscopic methods (NMR for diamagnetic species, EPR for paramagnetic ones, UV-Vis) for solution behavior.