of Industrial Chemistry reports that there are only three plants manufacturing pentacarbonyliron; BASF in Germany and GAF in Alabama have capacities of 9000 and 1500–2000 tonnes/year respectively. Most iron pentacarbonyl produced is decomposed on site to give pure carbonyl iron in analogy to carbonyl nickel. Some iron pentacarbonyl is burned to give pure iron oxide. Other uses of pentacarbonyliron are small in comparison. Reactions Decarbonylation and related CO substitution reactions Irradiation of Fe(CO)5 with UV produces Fe(CO)4, which captures a variety of ligands to give adducts. In the absence of trapping substrates, Fe2(CO)9 is produced. Many compounds are derived from Fe(CO)5 by substitution of CO by Lewis bases, L, to give derivatives Fe(CO)5−xLx. Common Lewis bases include isocyanides, tertiary phosphines and arsines, and alkenes. Usually these ligands displace only one or two CO ligands, but certain acceptor ligands such as PF3 and isocyanides can proceed to tetra- and pentasubstitution. These reactions are often induced with a catalyst or light. Illustrative is the synthesis of the bis(triphenylphosphine)iron tricarbonyl complex (Fe(CO)3(P(C6H5)3)2). In addition to the photochemical route, substitution can also induced by NaOH or NaBH4. The catalyst attacks a CO ligand, which labilizes another CO ligand toward substitution. The electrophilicity of Fe(CO)4L is less than that of Fe(CO)5, so the nucleophilic catalyst, disengages and attacks another molecule of Fe(CO)5. Oxidation and reduction Most metal carbonyls can be halogenated. Thus, treatment of Fe(CO)5 with iodines gives iron tetracarbonyl diiodide: Fe(CO)5 + I2 → Fe(CO)4I2 + CO Reduction of Fe(CO)5 with Na gives Na2Fe(CO)4, "tetracarbonylferrate" also called Collman's reagent. The dianion is isoelectronic with Ni(CO)4 but highly nucleophilic. Acid-base reactions Fe(CO)5 is not readily protonated, but it is attacked by hydroxide. Treatment of Fe(CO)5 with aqueous base produces [HFe(CO)4]−, via the metallacarboxylate intermediate. The oxidation of this monoanion gives triiron dodecarbonyl, Fe3(CO)12. Acidification of solutions of [HFe(CO)4]− gives iron tetracarbonyl dihydride, H2Fe(CO)4. Diene adducts Dienes react with Fe(CO)5 to give (diene)Fe(CO)3, wherein two CO ligands have been replaced by two olefins. Many dienes undergo this reaction, notably norbornadiene and 1,3-butadiene. One of the more historically significant derivatives is cyclobutadieneiron tricarbonyl (C4H4)Fe(CO)3, where C4H4 is the otherwise unstable cyclobutadiene. Receiving the greatest attention are complexes of the cyclohexadienes, the parent organic 1,4-dienes being available through the Birch reductions. 1,4-Dienes isomerize to the 1,3-dienes upon complexation. Fe(CO)5 reacts in dicyclopentadiene to form [Fe(C5H5)(CO)2]2, cyclopentadienyliron dicarbonyl dimer. This compound, called "Fp dimer" can be considered a hybrid