An iron hydride is a chemical system which contains
iron
Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...
and
hydrogen
Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
in some associated form.
[
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 atmosphere
The stellar atmosphere is the outer region of the volume of a star, lying above the stellar core, radiation zone and convection zone.
Overview
The stellar atmosphere is divided into several regions of distinct character:
* The photosphere, whi ...
s) or in small amounts at very low temperatures. The two elements form a metallic alloy
An alloy is a mixture of chemical elements of which in most cases at least one is a metal, metallic element, although it is also sometimes used for mixtures of elements; herein only metallic alloys are described. Metallic alloys often have prop ...
above of pressure, that has been advanced as a possible explanation for the low density of Earth's "iron" core.[ 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.][ 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).]
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 surviving for a while under ambient pressure, at temperatures below 150K.[
]
Binary compounds
Molecular compounds
* Hydridoiron (FeH). This molecule has been detected in the atmosphere of the Sun
The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...
and some red dwarf
A red dwarf is the smallest kind of star on the main sequence. Red dwarfs are by far the most common type of fusing star in the Milky Way, at least in the neighborhood of the Sun. However, due to their low luminosity, individual red dwarfs are ...
stars. It is stable only as a gas, above the boiling point of iron, or as traces in frozen noble gas
The noble gases (historically the inert gases, sometimes referred to as aerogens) are the members of Group (periodic table), group 18 of the periodic table: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn) and, in some ...
es below 30 K (where it may form complexes with molecular hydrogen, such as ).[ It may form a dimer and complexes with molecular hydrogen, such as and .][
* What was once believed to be trihydridoiron () was later shown to be FeH bound to molecular hydrogen .][
]
Polymeric network compounds
* Iron(I) hydride
Iron(I) hydride, systematically named iron hydride and poly(hydridoiron) is a solid inorganic compound with the chemical formula (also written or FeH). It is both thermodynamically and kinetically unstable toward decomposition at ambient temper ...
. It is stable at pressures exceeding 3.5 GPa.
* Iron(II) hydride
Iron(II) hydride, systematically named iron dihydride and poly(dihydridoiron) is solid inorganic compound with the chemical formula (also written )''n'' or ). ). It is kinetically unstable at ambient temperature, and as such, little is known abou ...
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
Iron pentahydride is a polyhydride, superhydride compound of iron and hydrogen, stable under high pressures. It is important because it contains atomic hydrogen atoms that are not bonded into smaller molecular clusters, and may be a superconductor ...
FeH5 is a polyhydride
A polyhydride or superhydride is a compound that contains an abnormally large amount of hydrogen. This can be described as high hydrogen stoichiometry. Examples include iron pentahydride , , and . By contrast, the more well known lithium hydride o ...
, 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.
Iron-hydrogen complexes
Complexes displaying iron–hydrogen bonds include, for example:
* iron tetracarbonyl hydride
Iron tetracarbonyl dihydride is the organometallic compound with the formula H2Fe(CO)4. This compound was the first transition metal hydride discovered. The complex is stable at low temperatures but decomposes rapidly at temperatures above –20 ...
FeH2(CO)4, the first such compound to be synthesised (1931).
Biological occurrence
Methanogens
Methanogens are anaerobic archaea that produce methane as a byproduct of their energy metabolism, i.e., catabolism. Methane production, or methanogenesis, is the only biochemical pathway for ATP generation in methanogens. All known methanogens b ...
, archaea
Archaea ( ) is a Domain (biology), domain of organisms. Traditionally, Archaea only included its Prokaryote, prokaryotic members, but this has since been found to be paraphyletic, as eukaryotes are known to have evolved from archaea. Even thou ...
, bacteria and some unicellular
A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms and ...
eukaryote
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
s contain hydrogenase
A hydrogenase is an enzyme that Catalysis, catalyses the reversible Redox, oxidation of molecular hydrogen (H2), as shown below:
Hydrogen oxidation () is coupled to the reduction of electron acceptors such as oxygen, nitrate, Ferric, ferric i ...
enzymes that catalyse
Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...
metabolic
Metabolism (, from ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cellular processes; the ...
reactions involving free hydrogen, whose active site is an iron atom with Fe–H bonds as well as other ligand
In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...
s.[
]
See also
* Iron–hydrogen alloy
References
[
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 ]
[
Hiroshi Nakazawa, Masumi Itazaki "Fe–H Complexes in Catalysis" Topics in Organometallic Chemistry (2011) 33: 27–81.
]
[
]
[
Helga Körsgen, Petra Mürtz, Klaus Lipus, Wolfgang Urban, Jonathan P. Towle, John M. Brown (1996), "The identification of the radical in the gas phase by infrared spectroscopy". ''The Journal of Chemical Physics'' volume 104(12) page 4859
]
[George V. Chertihin and Lester Andrews (1995), "Infrared spectra of FeH, , and in solid argon" ''Journal of Physical Chemistry'' volume 99, issue 32, pages 12131–12134
]
[A. S. Mikhaylushkin, N. V. Skorodumova, R. Ahuja, B. Johansson (2006)]
x (x=0.25; 0.50;0.75)"">"Structural and magnetic properties of FeHx (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