Fünfkohliges

Fünfkohliges is an adjective used in coordination chemistry to denote a species in which a central atom is bonded to five ligands. In German, the more common term for this situation is fünff koordiniert or fünfkoordiniert, while the form fÜnfkohliges appears mainly in older texts or as a direct calque from English and is not widely used in modern technical prose.

In five-coordinate complexes, the central atom can adopt different geometries. The two most common are trigonal bipyramidal and square pyramidal. Many five-coordinate species are fluxional in solution, interconverting between these geometries through a Berry pseudorotation mechanism, which can affect reactivity and spectroscopic properties. The geometry is influenced by the electronic configuration of the metal and the steric and electronic nature of the ligands.

Examples and occurrence: A classic five-coordinate complex is iron pentacarbonyl, Fe(CO)5, which adopts a trigonal bipyramidal arrangement in the gas phase. Five-coordinate species are widely observed among transition-metal carbonyls and other organometallic and inorganic complexes, including halide and phosphine or amine adducts. Their presence is often transient in catalytic cycles, where five-coordinate intermediates can participate in ligand binding, substrate activation, or migratory insertion steps. Detection and characterization rely on techniques such as X-ray crystallography, NMR spectroscopy, and EXAFS, complemented by computational studies to assess fluxional behavior.

See also: pentacoordination, coordination chemistry, Berry pseudorotation, fluxional molecule, five-coordinate complex.