Iron(II) hydride
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Iron(II) hydride, systematically named iron dihydride and poly(dihydridoiron) is solid
inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemist ...
with the
chemical formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...
(also written )''n'' or ). ). It is kinetically unstable at ambient temperature, and as such, little is known about its bulk properties. However, it is known as a black, amorphous powder, which was synthesised for the first time in 2014. Iron(II) hydride is the second simplest polymeric iron hydride (after
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 tempe ...
). Due to its instability, it has no practical industrial uses. However, in
metallurgical Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys. Metallurgy encompasses both the sc ...
chemistry, iron(II) hydride is fundamental to certain forms of iron-hydrogen alloys.


Nomenclature

The systematic name ''iron dihydride'', a valid
IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
name, is constructed according to the compositional nomenclature. However, as the name is compositional in nature, it does not distinguish between compounds of the same stoichiometry, such as molecular species, which exhibit distinct chemical properties. The systematic names ''poly(dihydridoiron)'' and ''poly errane(2)', also valid IUPAC names, are constructed according to the additive and electron-deficient substitutive nomenclatures, respectively. They do distinguish the titular compound from the others.


Dihydridoiron

Dihydridoiron, also systematically named ferrane(2), is a related inorganic compound with the chemical formula (also written ). It is both kinetically unstable at concentration and at ambient temperature. Dihydridoiron is the second simplest molecular iron hydride (after hydridoiron), and is also the progenitor of clusters with the same stoichiometry. In addition, it may be considered to be the iron(II) hydride monomer. It has been observed in
matrix isolation Matrix isolation is an experimental technique used in chemistry and physics. It generally involves a material being trapped within an unreactive matrix. A ''host'' matrix is a continuous solid phase in which ''guest'' particles (atoms, molecules, i ...
.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 ''The Journal of Chemical Physics'' is a scientific journal published by the American Institute of Physics that carries research papers on chemical physics.electron pair In chemistry, an electron pair or Lewis pair consists of two electrons that occupy the same molecular orbital but have opposite spins. Gilbert N. Lewis introduced the concepts of both the electron pair and the covalent bond in a landmark paper ...
, dihydridoiron has Lewis acidic character. Dihydridoiron has the capacity to capture up to four electron pairs from Lewis bases. A proton can join with the iron centre by dissociative protonation: : + → + Because dissociative protonation involves the capture of the
proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...
() to form a Kubas complex (]+) as an intermediate, dihydridoiron and its adducts of weak-field Lewis bases, such as water, also have Brønsted–Lowry basic character. They have the capacity to capture up to two protons. Its dissociated
conjugate acid A conjugate acid, within the Brønsted–Lowry acid–base theory, is a chemical compound formed when an acid donates a proton () to a base—in other words, it is a base with a hydrogen ion added to it, as in the reverse reaction it loses a ...
s are hydridoiron(1+) and iron(2+) ( and ). : + + + Aqueous solutions of adducts of weak-field Lewis bases are however, unstable due to hydrolysis of the dihydridoiron and hydridoiron(1+) groups: : + 2  → + 2  : + 3  → + + It should be expected that iron dihydride clusters and iron(II) hydride have similar acid-base properties, although reaction rates and equilibrium constants are different. Alternatively, a hydrogen centre in the dihydridoiron group in adducts of strong-field Lewis bases, such as carbon monoxide, may separate from the molecule by ionisation: : → + Because of this release of the proton, adducts of strong-field Lewis bases have may have Brønsted–Lowry acidic character. They have the capacity to release up to two protons. : + + Mixed adducts with Lewis bases of differing fields strengths may exhibit intermediate behaviour.


Structure

In iron(II) hydride, the atoms form a network, individual atoms being interconnected by
covalent bond A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atoms ...
s. Since it is a polymeric solid, a monocrystalline sample is not expected to undergo state transitions, such as melting and dissolution, as this would require the rearrangement of molecular bonds and consequently, change its chemical identity. Colloidal crystalline samples, wherein intermolecular forces are relevant, are expected to undergo state transitions. At least up to , iron(II) hydride is predicted to have a body-centred tetragonal crystalline structure with the I4/mmm space group. In this structure, iron centres have a capped square-antiprismatic coordination geometry, and hydrogen centres have square-planar and square-pyramidal geometries. An amorphous form of iron(II) hydride is also known. The infrared spectrum for dihydridoiron 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.


Electronic properties

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 electron In chemistry, an unpaired electron is an electron that occupies an orbital of an atom singly, rather than as part of an electron pair. Each atomic orbital of an atom (specified by the three quantum numbers n, l and m) has a capacity to contain ...
s (''X''5Δ''g'', ''A''5Π''g'', ''B''5Σ''g''+). With the first two excited states only 22 and 32 kJ mol−1 above the ground state, a sample of dihydridoiron contains trace quantities of excited states even at room temperature. Furthermore,
Crystal field theory Crystal field theory (CFT) describes the breaking of degeneracies of electron orbital states, usually ''d'' or ''f'' orbitals, due to a static electric field produced by a surrounding charge distribution (anion neighbors). This theory has been used ...
predicts that the low transition energies correspond to a colourless compound. The ground electronic state is 5Δg.


Metallurgical chemistry

In iron-hydrogen alloys that have hydrogen content near 3.48 wt%, hydrogen can precipitate as iron(II) hydride and lesser quantities of other polymeric iron hydrides. However, due to the limited solubility of hydrogen in iron, the optimum content for the formation of iron(II) hydride can only be reached by applying extreme pressure. In metallurgical chemistry, iron(II) hydride is fundamental to certain forms of iron-hydrogen alloys. It occurs as a brittle component within the solid matrix, with a physical makeup that depends on its formation conditions and subsequent heat treatment. As it decomposes over time, the alloy will slowly become softer and more ductile, and may start to suffer from
hydrogen embrittlement Hydrogen embrittlement (HE), also known as hydrogen-assisted cracking or hydrogen-induced cracking (HIC), is a reduction in the ductility of a metal due to absorbed hydrogen. Hydrogen atoms are small and can permeate solid metals. Once absorbed ...
.


Production

Dihydridoiron has been produced by several means, including: * By reaction of and PhMgBr under a hydrogen atmosphere (1929). * Electrical discharge in a mixture of pentacarbonyliron and dihydrogen diluted in helium at 8.5
Torr The torr (symbol: Torr) is a unit of pressure based on an absolute scale, defined as exactly of a standard atmosphere (). Thus one torr is exactly (≈ ). Historically, one torr was intended to be the same as one "millimeter of mercury ...
. * Evaporation of iron with a
laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...
in an atmosphere of hydrogen, pure or diluted in
neon Neon is a chemical element with the symbol Ne and atomic number 10. It is a noble gas. Neon is a colorless, odorless, inert monatomic gas under standard conditions, with about two-thirds the density of air. It was discovered (along with krypton ...
or argon, and condensing the products on a cold surface below 10 K. * Decomposition product of collision-excited
ferrocenium Ferrocenium tetrafluoroborate is an organometallic compound with the formula e(C5H5)2F4. This salt is composed of the cation e(C5H5)2sup>+ and the tetrafluoroborate anion (). The related hexafluorophosphate is also a popular reagent with simil ...
ions.


Iron reduction

Most iron(II) hydride is produced by iron reduction. In this process, stoichiometric amounts of iron and hydrogen react under an applied pressure of between approximately 45 and 75 GPa to produce iron(II) hydride according to the reaction: :''n''Fe + → The process involves iron(I) hydride as an intermediate, and occurs in two steps. #2''n''Fe + → # + →


Bis is(mesityl)ironreduction

Amorphous iron(II) hydride is produced by bis is(mesityl)ironreduction. In this process, bis is(mesityl)ironis reduced with hydrogen under an applied pressure of 100 atmospheres to produce iron(II) hydride according to the reaction: :''n''  + 4''n''  → + 4''n'' Hmes The process involves bis ydrido(mesityl)ironand dihydridoiron as intermediates, and occurs in three steps. # + → + 2 Hmes # + → + Hmes #''n''  →


Reactions

As dihydridoiron is an electron-deficient molecule, it spontaneously autopolymerises in its pure form, or converts to an adduct upon treatment with a Lewis base. Upon treatment of adducts of weak-field Lewis bases with a dilute standard acid, it converts to an hydridoiron(1+) salt and elemental hydrogen. Treatment of adducts of strong-field Lewis bases with a standard base, converts it to a metal ferrate(1−) salt and water. Oxidation of iron dihydrides give iron(II) hydroxide, whereas reduction gives hexahydridoferrate(4−) salts. Unless cooled to or below, dihydridoiron decomposes to produce elemental iron and hydrogen. Other iron dihydrides and adducts of dihydridoiron decompose at higher temperatures to also produce elemental hydrogen, and iron or polynuclear iron adducts: : → Fe +
Non-metals In chemistry, a nonmetal is a chemical element that generally lacks a predominance of metallic properties; they range from colorless gases (like hydrogen) to shiny solids (like carbon, as graphite). The electrons in nonmetals behave differentl ...
, including
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
, strongly attack iron dihydrides, forming hydrogenated compounds and iron(II) compounds: : + → FeO + Iron(II) compounds can also be prepared from an iron dihydride and an appropriate, concentrated acid: : + 2 HCl → + 2 


History

Even though complexes containing dihydridoiron was known since 1931, the simple compound with the molecular formula 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. Although iron(II) hydride has received attention only recently, complexes containing the dihydridoiron group have been known at least since 1931, when iron carbonyl hydride FeH2(CO)4 was first synthesised. The most precisely characterised FeH2L4 complex as of 2003 is FeH2(CO)2 (OPh)3sub>2. Complexes can also contain FeH2 with hydrogen molecules as a ligand. Those with one or two molecules of hydrogen are unstable, but FeH2(H2)3 is stable and can be produced by the evaporation of iron into hydrogen gas. From infrared spectra of samples of dihydridoiron trapped in frozen argon between 10 and 30 K, Chertihin and Andrews conjectured in 1995 that dihydridoiron readily dimerized into , and that it reacts with atomic hydrogen to produce trihydridoiron (). However, it was later proven that the product of the reaction was likely to have been hydrido(dihydrogen)iron ().


References

{{Hydrides by group Iron(II) compounds Metal hydrides