Following the original method,[4]photolysis of an acetic acid solution of Fe(CO)5 produces Fe2(CO)9 in good yield:[5][6]
2 Fe(CO)5 → Fe2(CO)9 + CO
Fe2(CO)9 consists of a pair of Fe(CO)3 centers linked by three bridging CO ligands. Although older textbooks show an Fe-Fe bond consistent with the 18 electron rule (8 valence electrons from Fe, two each from the terminal carbonyls, one each from the bridging carbonyls and one from the other Fe atom in the metal-metal bond), theoretical analyses have consistently indicated the absence of a direct Fe-Fe bond:[7] this latter model proposes an Fe-C-Fe three-center-two-electron "banana bond" for one of the bridging carbonyls. The minor isomer has been crystallized together with C60. The iron atoms are equivalent and octahedral molecular geometry. Elucidation of the structure of Fe2(CO)9 proved to be challenging because its low solubility inhibits growth of crystals. The Mößbauer spectrum reveals one quadrupole doublet, consistent with the D3h-symmetric structure.
Reactions
Fe2(CO)9 is a precursor to compounds of the type Fe(CO)4L and Fe(CO)3(diene). Such syntheses are typically conducted as tetrahydrofuran (THF) slurries. In these conversions, it is proposed that small amounts of Fe2(CO)9 dissolve according to the following reaction:[8]
Fe2(CO)9 → Fe(CO)5 + Fe(CO)4(THF)
Oxidative addition of allyl bromide to diiron nonacarbonyl gives the allyl iron(II) derivative:[9]
Fe2(CO)9 + BrCH2CH=CH2 → FeBr(CO)3(C3H5) + CO + Fe(CO)5
Fe2(CO)9 has also been employed in the synthesis of cyclopentadienones via a net [2+3]-cycloaddition from dibromoketones, known as the Noyori [3+2] reaction.[11]
Low temperature UV/vis photolysis of Fe2(CO)9 yields the Fe2(CO)8 unsaturated complex, producing both CO-bridged and unbridged isomers.[13]
Safety
Metal carbonyls are typically treated as if they are highly toxic.[5]
References
^Dewar, J., & Jones, H. O. (1907). On a New Iron Carbonyl, and on the Action of Light and of Heat on the Iron Carbonyls. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 79(527), 66–80. doi:10.1098/rspa.1907.0015
^Elschenbroich, C.; Salzer, A. ”Organometallics : A Concise Introduction” (2nd Ed) (1992) Wiley-VCH: Weinheim. ISBN3-527-28165-7
^Edmund Speyer; Hans Wolf (1924). "Über die Bildungsweise von Eisen-nonacarbonyl aus Eisen-pentacarbonyl". Berichte der Deutschen Chemischen Gesellschaft. 60 (6): 1424–1425. doi:10.1002/cber.19270600626.
^ abKing, R. B. Organometallic Syntheses. Volume 1 Transition-Metal Compounds; Academic Press: New York, 1965. ISBN0-444-42607-8.
^Jennifer C. Green, Malcolm L. H. Green, Gerard Parkin "The occurrence and representation of three-centre two-electron bonds in covalent inorganic compounds" Chem. Commun. 2012, 11481-11503. doi:10.1039/c2cc35304k
^F. Albert Cotton, Jan M. Troup "Reactivity of diiron nonacarbonyl in tetrahydrofuran. I. Isolation and characterization of pyridinetetracarbonyliron and pyrazinetetracarbonyliron" J. Am. Chem. Soc., 1974, volume 96, pp 3438–3443. doi:10.1021/ja00818a016
^Putnik, Charles F.; Welter, James J.; Stucky, Galen D.; d'Aniello, M. J.; Sosinsky, B. A.; Kirner, J. F.; Muetterties, E. L. (1978). "Metal clusters in catalysis. 15. A Structural and Chemical Study of a Dinuclear Metal Complex, Hexacarbonylbis(.eta.3-2-propenyl)diiron(Fe-Fe)". Journal of the American Chemical Society. 100 (13): 4107–4116. doi:10.1021/ja00481a020.
^Domingos, A. J. P.; Howell, J. A. S.; Johnson, B. F. G.; Lewis, J. (1990). "Reagents for the Synthesis of η-Diene Complexes of Tricarbonyliron and Tricarbonylruthenium". Inorg. Synth. Inorganic Syntheses. Vol. 28. pp. 52–55. doi:10.1002/9780470132593.ch11. ISBN9780470132593.
^Susan C. Fletcher; Martyn Poliakoff; James J. Turner (1986). "Structure and Reactions of Fe2(CO)8: An IR Spectroscopic study using 13C Photolysis with plane-polarized light, and matrix isolation". Inorg. Chem. 25 (20): 3597. doi:10.1021/ic00240a014.
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