Iron(II) hydride

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Iron(II) hydride
Identifiers
CAS number 33485-98-2
PubChem 141155
ChemSpider 124509
Jmol-3D images Image 1
Properties
Molecular formula FeH24•
Molar mass 57.861 g mol−1
Related compounds
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)
Infobox references

Iron(II) hydride (systematically named iron dihydride) is an inorganic compound with the chemical formula (FeH
2
)
n
(also written ([FeH
2
]
)n or FeH
2
). It is a solid, known only as a black, amorphous powder, which was synthesised for the first time in 2014.[1][dubious ]

Nomenclature[edit]

The systematic name iron dihydride, a valid IUPAC name, is constructed according to the compositional nomenclature. Iron dihydride is also used to refer to the related molecular compound dihydridoiron and its oligomers. Care should be taken to avoid confusing the two compounds.

Chemical properties[edit]

In iron(II) dihydride, the atoms form a network (polymer), being connected by covalent bonds.[1] Other lower metal hydrides polymerise in a similar fashion. As of 2014, the only known method of synthesising iron dihydride, is the hydrogenation of bis(mesityl)iron under elevated pressure.[1] The amorphous form is air and water sensitive.[1]

Dihydridoiron[edit]

Dihydridoiron is a related compound with the chemical formula FeH
2
(also written [FeH
2
]
). It is a gas that cannot persist undiluted. Unsolvated dihydridoiron will spontaneously autopolymerise to oligomers. It has been observed in matrix isolation.[2] 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.

History[edit]

Even though complexes containing dihydridoiron was known since 1931,[3] the simple compound with the molecular formula FeH
2
is only a much more recent discovery. Following the discovery of the first complex containing dihydridoiron, tetracarbonylate, it was also quickly discovered that it is not possible to remove the carbon monoxide by thermal means - heating an dihydridoiron containing 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 adducts.

Chemical Properties[edit]

Radicality[edit]

A few of dihydridoiron's electronic states lie relatively close to each other, giving rise to varying degrees of radical chemistry. The ground state and the first two excited states are all quintet radicals with four unpaired electrons (X5Δg, A5Πg, B5Σg+). With the first two excited states only 25 and 27 kJ mol−1 above the ground state, a sample of dihydridoiron exists as a mixture of electronic states even at room temperature, giving rise to complex reactions.

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.[2] The ground electronic state is 5Δg.[2]

State transitions of 56FeH2 in the ν3 fundamental band[2]
Transition Wavenumber
cm−1
Frequency
THz
P4(10) 1614.912 48.4100
P4(7) 1633.519 48.9717
Q4(4),Q3(3) 1672.658 50.1450
Q4(4),Q4(5),Q3(3) 1676.183 50.2507
R4(4) 1704.131 51.0886
R4(5) 1707.892 51.2013
R4(8) 1725.227 51.7210
R4(9) 1729.056 52.8358

Acidity[edit]

The two-coordinate hydridoiron group (-FeH) in hydridoirons such as dihydridoiron can accept an electron-pair donating ligand into the molecule by adduction:[4]

[FeH
2
]
+ L → [FeH
2
L]

Because of this acceptance of the electron-pair donating ligand (L), dihydridoiron has Lewis-acidic character. Dihydridoiron can accept four electron-pairs from ligands, as in the case of tetracarbonyldihydridoiron (FeH
2
(CO)
4
).

Chemical reactions[edit]

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.

Samples trapped in frozen argon at 10 K were apparently unaffected by annealing to 30 K.[5] However the compound survives transiently at above cryogenic temperatures.

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
2
H
4
, and that it would react with atomic hydrogen to produce FeH
3
.[5] However the identity of the latter has been questioned.[4]

Production[edit]

Dihydridoiron has been produced by several means, including:

  • By reaction of FeCl
    2
    and PhMgBr under a hydrogen atmosphere (1929).[citation needed]
  • Electrical discharge in a mixture of pentacarbonyliron and dihydrogen diluted in helium at 8.5 Torr.[2]
  • 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.[5][4]
  • Decomposition product of collision-excited ferrocenium ions.[6]

References[edit]

  1. ^ a b c d Morris, Leah; Trudeau, Michel L.; Lees, Martin R.; Hanna, John V.; Antonelli, David M. (25 March 2014). "On the Path to Bulk FeH
    2
    : Synthesis and Magnetic Properties of Amorphous Iron(II) Hydride". Journal of Alloys and Compounds (Elsevier Ltd.) 590: 199–204. doi:10.1016/j.jallcom.2013.12.099.
     
  2. ^ a b c d e 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
  3. ^ Hieber, W.; Leutert, F. (1 April 1931). "Zur kenntnis des koordinativ gebundenen kohlenoxyds: Bildung von eisencarbonylwasserstoff". Naturwissenschaften (PDF) (in German) (Springer-Verlag) 19 (17): 360–361. doi:10.1007/BF01522286. ISSN 1432-1904. 
  4. ^ a b c 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
  5. ^ a b c 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
  6. ^ 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.