Fluxional organometallic compounds are organometallic compounds in which the ligands or atoms undergo rapid and continuous rearrangement within the molecule, so that different nuclei exchange their positions on the NMR time scale. Such molecules are therefore described as fluxional or dynamically mobile compounds. Although a molecule may possess several chemically non-equivalent positions at any particular instant, rapid intramolecular rearrangement can make these positions equivalent when observed by NMR spectroscopy. A classic example is iron pentacarbonyl, Fe(CO)₅. Its structure is approximately trigonal bipyramidal, containing two axial and three equatorial carbonyl groups. If the molecule were completely rigid, the axial and equatorial CO groups would be chemically different and would be expected to give separate signals in an NMR spectrum. However, Fe(CO)₅ undergoes rapid intramolecular exchange of the carbonyl groups through a process commonly described as Berry pseudorotation. During this process, axial and equatorial carbonyl groups interchange their positions. The fluxional behaviour of Fe(CO)₅ can be demonstrated by variable-temperature NMR spectroscopy. At ordinary temperatures, the rate of exchange of CO groups is very high compared