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''Iron hydride''' is a generic name for [[chemical compounds]] of [[iron]] and [[hydrogen]].

Because of the common occurrence of those two [[element (chemistry)|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.<ref name=Mikhay/><ref name=Badding/>

A few molecular compounds have been detected in extreme environments (such as [[stellar atmosphere]]s) or isolated in small amounts at very low temperatures. The two elements form a metallic [[alloy]] above 35.000 atmospheres of pressure, that has been advanced as a possible explanation for the low density of [[inner core|Earth's "iron" core]].<ref name=Badding/><ref name=Saxena/>  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 complex]]es where iron is bound to hydrogen (among other elements).<ref name=Nakazawa/><ref name="Hieber31"/>

Small amounts of hydrogen (up to about 0.08% by weight) may be absorbed into iron as it solidifies from molten state.<ref name=Mikhay/> 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 is stable only as a gas, above the boiling point of iron.  It has been detected in the atmosphere of the [[Sun]] and some [[red dwarf]] stars.
* [[Iron(II) hydride]] or ferrous hydride ({{chem|FeH|2}}).  This compound has been isolated very low temperatures, and decomposes into the elements on warming.<ref name="Körsgen1996"/><ref name=Chertihin/>
* FeH<sub>2</sub>(H<sub>2</sub>)<sub>3</sub>. This stable compound can be produced by the evaporation of iron into hydrogen gas.<ref name="Wang2009"/>
* Metallic [[Iron-hydrogen alloy]]s. 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 [[gigapascal|GPa]] even at high temperatures and that is reported to survive for a while under ambient pressure, at temperatures below 150K.<ref name=Antonov/>

* What was once believed to be "Iron(III) hydride" or "ferric hydride"({{chem|FeH|3}}) was later shown to be FeH bound to molecular hydrogen H<sub>2</sub>.<ref name=Andrews2004/>

===Compounds with other elements===
Complexes displaying iron-hydrogen bonds are more accessible. Some examples are 
* [[iron tetracarbonyl hydride]] FeH<sub>2</sub>(CO)<sub>4</sub>, the first such compound to be synthesised (1931).<ref name="Hieber31"/>
* FeH<sub>2</sub>(CO)<sub>2</sub>[P(OPh)<sub>3</sub>]<sub>2</sub>, the most precisely characterised FeH<sub>2</sub>L<sub>4</sub> complex (as of 2003).
* Salts of the {{chem|FeH|6|2-}} anion, such as [[magnesium iron hexahydride]], {{chem|MgFeH|6}}, produced by treating mixtures of magnesium and iron powders with high pressures of H<sub>2</sub>.

Complexes can also contain FeH<sub>2</sub> with hydrogen molecules as a ligand.  Those with one or two molecules of hydrogen are unstable, 

[[Methanogens]], [[archaea]], [[bacteria]] and some [[unicellular eukaryote]]s contain [[hydrogenase|hydrogenase enzyme]]s that [[catalysis|catalyse]] [[metabolism|metabolic]] reactions involving free hydrogen, whose active site is an iron atom with Fe-H bonds as well as other [[ligand]]s.<ref name=Font/>

== References ==
<references>

<ref name=Font>
  J. C. Fontecilla-Camps, P. Amara, C. Cavazza, Y. Nicolet and A. Volbeda (2009), "Structure-function relationships of anaerobic gas-processing metalloenzymes", Nature, volume 460, pages 814-822.{{doi|10.1038/nature08299}}
</ref>

<ref name="Hieber31">
  {{cite journal|last=Hieber|first=W.|coauthors=F. Leutert|year=1931|journal=Naturwissenschaften|volume=18|issue=32|page=360}}
</ref>

<ref name="Körsgen1996">
  Helga Körsgen, Petra Mürtz, Klaus Lipus, Wolfgang Urban, Jonathan P. Towle, John M. Brown (1996), "The identification of the {{chem|FeH|2}} radical in the gas phase by infrared spectroscopy". The Journal of Chemical Physics, volume 104, issue 12, page 4859 ISSN 00219606 {{doi|10.1063/1.471180}}
</ref>

<ref name=Badding>
  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}}
</ref>

<ref name=Nakazawa>
  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}}
</ref>

<ref name="Wang2009">
   Xuefeng Wang and Lester Andrews (2009), "Infrared Spectra and Theoretical Calculations for Fe, Ru, and Os Metal Hydrides and Dihydrogen Complexes". The Journal of Physical Chemistry A, volume 113, issue 3, pages 551–563 issn:1089-5639 {{doi|10.1021/jp806845h}}
</ref>

<ref name=Chertihin>
  George V. Chertihin and Lester Andrews (1995), "Infrared spectra of FeH, {{chem|FeH|2}}, and {{chem|FeH|3}} in solid argon". Journal of Physical Chemistry, volume 99, issue 32, pages 12131–12134 {{doi|10.1021/j100032a013}}
</ref>

<ref name=Mikhay>
  A. S. Mikhaylushkin, N. V. Skorodumova, R. Ahuja, B. Johansson (2006), [http://proceedings.aip.org/resource/2/apcpcs/837/1/161_1 "Structural and magnetic properties of FeH<sub>x</sub> (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}}
</ref>

<ref name=Antonov>
  V. E. Antonov, K. Cornell, V.K. Fedotov, A. I. Kolesnikov E.G. Ponyatovsky, V.I. Shiryaev, H. Wipf (1998) [http://issp3.issp.ac.ru/lhpp/PapersAntonov/114.pdf "Neutron diffraction investigation of the dhcp and hcp iron hydrides and deuterides"]. Journal of Alloys and Compounds, volume 264, pages 214–222 {{doi|10.1016/S0925-8388(97)00298-3}}
</ref>

<ref name=Surendra>
  Surendra K. Saxena, Hanns-Peter Liermann, and Guoyin Shen (2004), "Formation of iron hydride and high-magnetite at high pressure and temperature". Physics of the Earth and Planetary Interiors, volume 146, pages 313-317. {{doi|10.1016/j.pepi.2003.07.030}}
</ref>

<ref name=Andrews2004>
  Lester Andrews (2004), "Matrix infrared spectra and density functional calculations of transition metal hydrides and dihydrogen complexes" Chemical Society Reviews, volume 33, issue 2, pages 123–132 {{doi|10.1039/B210547K}}
</ref>

</references>
{{reflist|colwidth=33em}}

[[Category:Metal hydrides]]
[[Category:Ferrous alloys]]

[[fr:Hydrure de fer]]