to the excitement arising from the discovery of ferrocene and its many analogues. Ferrocene and its numerous derivatives have no large-scale applications, but have many niche uses that exploit their unusual structure (ligand scaffolds, pharmaceutical candidates), robustness (anti-knock formulations, precursors to materials), and redox reactions (reagents and redox standards). Use for global cooling has been proposed. Ferrocene and its derivatives are antiknock agents added to the petrol used in motor vehicles, and are safer than the now-banned tetraethyllead. Petrol additive solutions containing ferrocene can be added to unleaded petrol to enable its use in vintage cars designed to run on leaded petrol. The iron-containing deposits formed from ferrocene can form a conductive coating on the spark plug surfaces. Iron pentacarbonyl Iron pentacarbonyl, also known as iron carbonyl, is the compound with formula . Under standard conditions Fe(CO)5 is a free-flowing, straw-colored liquid with a pungent odour. This compound is a common precursor to diverse iron compounds, including many that are useful in organic synthesis. Fe(CO)5 is prepared by the reaction of fine iron particles with carbon monoxide. Fe(CO)5 is inexpensively purchased. Iron pentacarbonyl is one of the homoleptic metal carbonyls; i.e. metal complexes bonded only to CO ligands. Other examples include octahedral Cr(CO)6 and tetrahedral Ni(CO)4. Most metal carbonyls have 18 valence electrons, and Fe(CO)5 fits this pattern with 8 valence electrons on Fe and five pairs of electrons provided by the CO ligands. Reflecting its symmetrical structure and charge neutrality, Fe(CO)5 is volatile; it is one of the most frequently encountered liquid metal complexes. Fe(CO)5 adopts a trigonal bipyramidal structure with the Fe atom surrounded by five CO ligands: three in equatorial positions and two axially bound. The Fe-C-O linkages are each linear. Fe(CO)5 is the archetypal fluxional molecule due to the rapid interchange of the axial and equatorial CO groups via the Berry mechanism on the NMR timescale. Consequently, the13C NMR spectrum exhibits only one signal due to the rapid interchange between nonequivalent CO sites. In Europe, iron pentacarbonyl was once used as an anti-knock agent in petrol in place of tetraethyllead. Two more modern alternative fuel additives are ferrocene and methylcyclopentadienyl manganese tricarbonyl. Fe(CO)5 is used in the production of "carbonyl iron", a finely divided form of iron used in magnetic cores of high-frequency coils for electronics, and for manufacture of the active ingredients of some radar absorbent materials (e.g. iron ball paint). It is famous as