of ferrocene and Fe(CO)5, although in terms of its reactivity, it resembles neither. CO substitution reactions Upon UV irradiation Fe(CO)5 absorbs light population and metal-to-CO charge transfer band inducing CO photolysis and generating singlet and triplet coordinatively unsaturated intermediate Fe(CO)4 with high quantum yield. Prolonged irradiation in gas phase may proceed to further CO detach until atomic Fe formation. Other uses In Europe, iron pentacarbonyl was once used as an anti-knock agent in petrol in place of tetraethyllead; it was produced by IG Farben and commercially marketed under the trade names, “Motolin” and “Monopolin”. 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 Fe, a material used in magnetic cores of high-frequency coils for radios and televisions and for manufacture of the active ingredients of some radar absorbent materials (e.g. iron ball paint). It is famous as a chemical precursor for the synthesis of various iron-based nanoparticles. Iron pentacarbonyl has been found to be a strong flame speed inhibitor in oxygen based flames. A few hundred ppm of iron pentacarbonyl are known to reduce the flame speed of stoichiometric methane–air flame by almost 50%. However due to its toxic nature it has not been used widely as a flame retardant. Toxicity and hazards Fe(CO)5 is toxic, which is of concern because of its volatility (vapour pressure: at 20 °C). If inhaled, iron pentacarbonyl may cause lung irritation, toxic pneumonitis, or pulmonary edema. Like other metal carbonyls, Fe(CO)5 is flammable. It is, however, considerably less toxic than nickel tetracarbonyl. The National Institute for Occupational Safety and Health has set a recommended exposure limit for iron pentacarbonyl at 0.1 ppm (0.23 mg/m3) over an eight-hour time-weighted average, and a short-term exposure limit at 0.2 ppm (0.45 mg/m3). References Carbonyl complexes Iron(0) compounds Antiknock agents