Iron hydride

Ball-and-stick diagram of the iron(I) hydride (FeH) free molecule.

An iron hydride is a chemical system which contains iron and hydrogen in some associated form, either as compounds or mixtures, as in the case of alloys.

Because of the common occurrence of those two elements in the universe, possible compounds of hydrogen and iron have attracted the attention of many chemists and physicists. However, the two elements do not combine in ordinary conditions.^[1][2]

A few molecular compounds have been detected in extreme environments (such as stellar atmospheres) or isolated in small amounts at very low temperatures. The two elements form a metallic alloy above 35000 atmospheres 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.

On the other hand, there are many fairly stable hydride complexes where iron is bound to hydrogen (among other elements).^[4][5]

Small amounts of hydrogen (up to about 0.08% by weight) may be absorbed into iron as it solidifies from molten state.^[1] This impurity does not change iron's crytalline structure, but can affect its mechanical properties.

Overview

Binary compounds

Only a few compounds containing exclusively the two elements have been reported in technical literature, including:

• Iron(I) hydride (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·H
  2.^[6]
• Iron(II) hydride or ferrous hydride (FeH
  2). This compound has been obtained only in very rarefied gases or trapped in frozen gases below 30 K, and decomposes into the elements on warming.^[7][8] It may form a dimer Fe
  2H
  4 and complexes with molecular hydrogen, such as FeH₂(H₂)₂ and FeH₂(H₂)₃.^[6][9]
• Metallic Iron–hydrogen alloys. These include iron solidified from the melt at ordinary pressures, that may incorporate small amounts of hydrogen as an interstitial compound; and an intermetallic compound with formula FeH, stable at pressures above 3.5 GPa even at high temperatures and that is reported to survive for a while under ambient pressure, at temperatures below 150K.^[10]
• What was once believed to be "Iron(III) hydride" or "ferric hydride"(FeH
  3) was later shown to be FeH bound to molecular hydrogen H₂.^[9]

Compounds with other elements

Complexes displaying iron–hydrogen bonds are more accessible. Examples are

• iron tetracarbonyl hydride FeH₂(CO)₄, the first such compound to be synthesised (1931).^[5]
• FeH₂(CO)₂[P(OPh)₃]₂, the most precisely characterised FeH₂L₄ complex (as of 2003).
• 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.

Complexes can also contain FeH₂ with hydrogen molecules as a ligand. Those with one or two molecules of hydrogen are unstable,

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.^[11]

See also

• Iron–hydrogen alloy

References

1. A. S. Mikhaylushkin, N. V. Skorodumova, R. Ahuja, B. Johansson (2006), "Structural and magnetic properties of FeH_x (x=0.25; 0.50;0.75)". 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
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. doi:10.1016/j.pepi.2003.07.030.
4. 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
5. Hieber, W.; F. Leutert (1931). "Zur Kenntnis des koordinativ gebundenen Kohlenoxyds: Bildung von Eisencarbonylwasserstoff". Naturwissenschaften 19 (17): 360–361. doi:10.1007/BF01522286.
6. 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. doi:10.1021/jp806845h.
7. 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
8. 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
9. 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.
10. 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.
11. 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. doi:10.1038/nature08299. PMID 19675641.