Iron hydride

An iron hydride is a chemical system which contains iron and hydrogen in some associated form.[1][2]

Space-filling model of the iron(I) hydride (FeH) free molecule.

Because of the common occurrence of those two elements in the universe, possible compounds of hydrogen and iron have attracted attention. A few molecular compounds have been detected in extreme environments (such as stellar atmospheres) or in small amounts at very low temperatures. The two elements form a metallic alloy above 35,000 standard atmospheres (3.5 GPa) of pressure, that has been advanced as a possible explanation for the low density of Earth's "iron" core.[2][3] However those compounds are unstable when brought to ambient conditions, and eventually decompose into the separate elements.

Small amounts of hydrogen (up to about 0.08% by weight) are absorbed into iron as it solidifies from its molten state.[4] Although the H2 is simply an impurity, its presence can affect the material's mechanical properties.

Despite the fleeting nature of binary iron hydrides, there are many fairly stable complexes containing iron-hydrogen bonds (and other elements).[5][6]

Overview

Solid solutions

Iron and iron-based alloys can form solid solutions with hydrogen, which under extreme pressure may reach stoichiometric proportions, remaining stable even at high temperatures and that is reported to survive for a while under ambient pressure, at temperatures below 150K.[7]

Molecular compounds
  • Hydridoiron (FeH). This molecule has been detected in the atmosphere of the Sun and some red dwarf stars. It is stable only as a gas, above the boiling point of iron, or as traces in frozen noble gases below 30 K (where it may form complexes with molecular hydrogen, such as FeH·H2).[8]
  • Dihydridoiron (FeH2). This compound has been obtained only in rarefied gases or trapped in frozen gases below 30 K (−243 °C), decomposing into the elements on warming.[9][10] It may form a dimer Fe2H4 and complexes with molecular hydrogen, such as FeH2(H2)2 and FeH2(H2)3.[8][11]
  • What was once believed to be trihydridoiron (FeH3) was later shown to be FeH bound to molecular hydrogen H2.[11]

Polymeric network compounds
  • Iron(I) hydride. It is stable at pressures exceeding 3.5 GPa.
  • Iron(II) hydride or ferrous hydride. It is stable at pressures between 45 and 75 GPa.
  • Iron(III) hydride or ferric hydride. It is stable at pressures exceeding 65 GPa.
  • Iron pentahydride FeH5 is a polyhydride, where there is more hydrogen than expected by valence rules. It is stable under pressures over 85 GPa. It contains alternating sheets of FeH3 and atomic hydrogen.[12]

Iron-hydrogen complexes

Complexes displaying iron–hydrogen bonds include, for example:

  • iron tetracarbonyl hydride FeH2(CO)4, the first such compound to be synthesised (1931).[6]
  • FeH2(CO)2[P(OPh)3]2.
  • Salts of the FeH2−
    6     anion, such as magnesium iron hexahydride, MgFeH
    6    , produced by treating mixtures of magnesium and iron powders with high pressures of H2.
  • Di- and polyiron hydrides, e.g. [HFe2(CO)8]− and the cluster [HFe3(CO)11]−.

Complexes are also known with molecular hydrogen (H
2) ligands.

Biological occurrence

Methanogens, archaea, bacteria and some unicellular eukaryotes contain hydrogenase enzymes that catalyse metabolic reactions involving free hydrogen, whose active site is an iron atom with Fe–H bonds as well as other ligands.[13]

See also

References
  1. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  2. J.V. Badding, R.J. Hemley, and H.K. Mao (1991), "High-pressure chemistry of hydrogen in metals: in situ study of iron hydride." Science, American Association for the Advancement of Science, volume 253, issue 5018, pages 421–424 doi:10.1126/science.253.5018.421
  3. Saxena, Surendra K.; Liermann, Hanns-Peter; Shen, Guoyin (2004). "Formation of iron hydride and high-magnetite at high pressure and temperature". Physics of the Earth and Planetary Interiors. 146 (1–2): 313–317. Bibcode:2004PEPI..146..313S. doi:10.1016/j.pepi.2003.07.030.
  4. A. S. Mikhaylushkin, N. V. Skorodumova, R. Ahuja, B. Johansson (2006), "Structural and magnetic properties of FeHx (x=0.25; 0.50;0.75)" Archived 23 February 2013 at archive.today. In: Hydrogen in Matter: A Collection from the Papers Presented at the Second International Symposium on Hydrogen in Matter (ISOHIM), AIP Conference Proceedings, volume 837, pages 161–167 doi:10.1063/1.2213072
  5. Hiroshi Nakazawa, Masumi Itazaki "Fe–H Complexes in Catalysis" Topics in Organometallic Chemistry (2011) 33: 27–81. doi:10.1007/978-3-642-14670-1_2
  6. Hieber, W.; F. Leutert (1931). "Zur Kenntnis des koordinativ gebundenen Kohlenoxyds: Bildung von Eisencarbonylwasserstoff". Naturwissenschaften. 19 (17): 360–361. Bibcode:1931NW.....19..360H. doi:10.1007/BF01522286. S2CID 791569.
  7. Antonov, V. E.; Cornell, K.; Fedotov, V.K.; Ponyatovsky, A. I. Kolesnikov E.G.; Shiryaev, V.I.; Wipf, H. (1998). "Neutron diffraction investigation of the dhcp and hcp iron hydrides and deuterides" (PDF). Journal of Alloys and Compounds. 264 (1–2): 214–222. doi:10.1016/S0925-8388(97)00298-3.
  8. Wang, Xuefeng; Andrews, Lester (2009). "Infrared Spectra and Theoretical Calculations for Fe, Ru, and Os Metal Hydrides and Dihydrogen Complexes". The Journal of Physical Chemistry A. 113 (3): 551–563. Bibcode:2009JPCA..113..551W. doi:10.1021/jp806845h. PMID 19099441.
  9. Helga Körsgen, Petra Mürtz, Klaus Lipus, Wolfgang Urban, Jonathan P. Towle, John M. Brown (1996), "The identification of the FeH
    2     radical in the gas phase by infrared spectroscopy". The Journal of Chemical Physics volume 104(12) page 4859 doi:10.1063/1.471180
  10. George V. Chertihin and Lester Andrews (1995), "Infrared spectra of FeH, FeH
    2    , and FeH
    3     in solid argon" Journal of Physical Chemistry volume 99, issue 32, pages 12131–12134 doi:10.1021/j100032a013
  11. Andrews, Lester (2004). "Matrix infrared spectra and density functional calculations of transition metal hydrides and dihydrogen complexes". Chemical Society Reviews. 33 (2): 123–132. doi:10.1039/B210547K. PMID 14767507.
  12. Pépin, C. M.; Geneste, G.; Dewaele, A.; Mezouar, M.; Loubeyre, P. (27 July 2017). "Synthesis of FeH5 : A layered structure with atomic hydrogen slabs". Science. 357 (6349): 382–385. Bibcode:2017Sci...357..382P. doi:10.1126/science.aan0961. PMID 28751605.
  13. Fontecilla-Camps, J. C.; Amara, P.; Cavazza, C.; Nicolet, Y.; Volbeda, A. (2009). "Structure-function relationships of anaerobic gas-processing metalloenzymes". Nature. 460 (7257): 814–822. Bibcode:2009Natur.460..814F. doi:10.1038/nature08299. PMID 19675641. S2CID 4421420.
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