|Jmol-3D images||Image 1|
|Molar mass||57.861 g mol−1|
|Related compounds||iron hydride FeH FeH3|
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
Iron(II) hydride (systematically named iron dihydride), also called ferrous hydride, is an inorganic compound with the chemical formula FeH
2. To date, it has only been observed in the molecular form, which is to say that its bulk properties (including its standard state) remain unknown. Molecular iron(II) hydride and its adducts only persists at cryogenic temperatures. It has four unpaired electrons and its groundstate formula may be written FeH4•
2 to emphasize that fact.
Iron(II) hydride has been isolated only in small quantities, in rarefied gas environment or trapped in frozen inert gases at extremely low temperatures, and dissociates into the elements in ambient conditions. It is one of the few known compounds of iron and hydrogens, and the lightest dihydride of an element from group 8 of the periodic table.
Even though derivatives of iron(II) hydride was known since 1931, the simple compound with the empirical formula FeH
2 is only a much more recent discovery. Following the discovery of the first derivative of iron(II) hydride, tetracarbonylate, it was also quickly discovered that it is not possible to remove the carbon monoxide by thermal means - heating an iron(II) hydride complex only causes it to decompose, a habit attributable to the weak iron-hydrogen bond. Thus, a practical method has been sought since then for the production of the pure compound, without the involvement of a liquid phase. Furthermore, there is also on going research into its other derivatives and adducts.
Iron(II) hydride has been produced by several means, including:
- By reaction of FeCl
2 and PhMgBr under a hydrogen atmosphere (1929).
- Electrical discharge in a mixture of pentacarbonyliron and dihydrogen diluted in helium at 8.5 Torr.
- Evaporation of iron with a laser in an atmosphere of hydrogen, pure or diluted in neon or argon, and condensing the products on a cold surface below 10 K.
- Decomposition product of collision-excited ferrocenium ions.
The infrared spectrum shows that the molecule has a linear H−Fe−H structure in the gas phase, with an equilibrium distance between the iron atom and the hydrogen atoms of 0.1665 nm. The ground electronic state is 5Δg.
Due to the instability of this molecule and the low temperature conditions in which it is produced, and the fact that it was produced during laser ablation with two other species, FeH and FeH3, there is little known about its chemistry.
FeH2 is thermodynamically unstable with respect to the loss of a hydrogen atom
Samples trapped in frozen argon at 10 K were apparently unaffected by annealing to 30 K. However the compound survives transiently at above cryogenic temperatures. It decomposes, by successively releasing the two hydrogen atoms, into another similarly unstable radical, FeH, and then into of iron in the form of nanocrystals.
From infrared spectra of samples trapped in frozen argon between 10 and 30 K, Chertihin and Andrews conjectured in 1995 that FeH
2 readily dimerized into Fe
4, and that it would react with atomic hydrogen to produce FeH
3. However the identity of the latter has been questioned.
- 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, issue 12, page 4859 ISSN 00219606 doi:10.1063/1.471180
- Hieber, W.; Leutert, F. (1 April 1931). "Zur kenntnis des koordinativ gebundenen kohlenoxyds: Bildung von eisencarbonylwasserstoff". Naturwissenschaften (PDFdoi:10.1007/BF01522286. ISSN 1432-1904.) (in German) (Springer-Verlag) 19 (17): 360–361.
- 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
- 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
- Rod S. Mason and Lara J. Kelly (2012), "Synthesis of protonated ferrocene isomers in the gas phase and their study by mass spectrometry". Arkivoc, volume 2012, issue 7, pages 137-157.