Iron pentacarbonyl, also known as iron carbonyl, is the compound with formula ()5. 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.
Iron carbonyl is sometimes confused with carbonyl iron, a high-purity metal prepared by decomposition of iron pentacarbonyl.
Photolysis of Fe(CO)5 produces Fe2(CO)9, a yellow-orange solid, also described by Mond. When heated, Fe(CO)5 converts to small amounts of the metal cluster Fe3(CO)12, a green solid, although simple thermolysis is not a useful synthesis (see below). Each iron carbonyl exhibits distinct reactivity.
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 of ferrocene and Fe(CO)5, although in terms of its reactivity, it resembles neither.
Iron pentacarbonyl has been found to be a strong flame speed inhibitor in oxygen based flames. Few hundred ppm of iron pentacarbonyl are known to reduce the flame speed of stoichiometric methane-air flame by al most 50%. However due to its toxic nature it has not been used widely as flame retardant.
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