| Names | |
|---|---|
| Systematic IUPAC name Dihydridoiron(4•) | |
| Identifiers | |
3D model (JSmol) | |
| ChemSpider | |
PubChem CID | |
| |
| |
| Properties | |
| FeH24• | |
| Molar mass | 57.861 g mol−1 |
| Related compounds | |
Related compounds | iron hydrides, FeH, FeH3 |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). | |
Iron(II) hydride, systematically named iron dihydride and poly(dihydridoiron) is solid inorganic compound with the chemical formula (FeH
2)
n (also written ([FeH
2])n or FeH
2). ). 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. [1]
Iron(II) hydride is the second simplest polymeric iron hydride (after iron(I) hydride). Due to its instability, it has no practical industrial uses. However, in metallurgical chemistry, iron(II) hydride is fundamental to certain forms of iron-hydrogen alloys.
The systematic name iron dihydride, a valid IUPAC 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[ferrane(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, also systematically named ferrane(2), is a related inorganic compound with the chemical formula FeH
2 (also written [FeH
2]). 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. [2]
An electron pair of a Lewis base can join with the iron centre in dihydridoiron by adduction:
Because of this capture of an adducted electron pair, 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 (H+
) to form a Kubas complex ([FeH(H
2)]+) 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 acids are hydridoiron(1+) and iron(2+) (FeH+
and Fe2+
).
Aqueous solutions of adducts of weak-field Lewis bases are however, unstable due to hydrolysis of the dihydridoiron and hydridoiron(1+) groups:
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. [3]
In iron(II) hydride, the atoms form a network, individual atoms being interconnected by covalent bonds. 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. [4]
At least up to −173 °C (−279 °F), 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.
| 9-Coordinate Fe-centre | 4-Coordinate H-centre | 5-Coordinate H-centre |
|---|---|---|
 |  |  |
An amorphous form of iron(II) hydride is also known. [1]
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. [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 |
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 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 predicts that the low transition energies correspond to a colourless compound.
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. [5] 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. [5]
Dihydridoiron has been produced by several means, including:
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:
The process involves iron(I) hydride as an intermediate, and occurs in two steps.
Amorphous iron(II) hydride is produced by bis[bis(mesityl)iron] reduction. In this process, bis[bis(mesityl)iron] is reduced with hydrogen under an applied pressure of 100 atmospheres to produce iron(II) hydride according to the reaction:
The process involves bis[hydrido(mesityl)iron] and dihydridoiron as intermediates, and occurs in three steps.
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 −243 °C (−405.4 °F) or below, dihydridoiron decomposes to produce elemental iron and hydrogen. [7] Other iron dihydrides and adducts of dihydridoiron decompose at higher temperatures to also produce elemental hydrogen, and iron or polynuclear iron adducts:
Non-metals, including oxygen, strongly attack iron dihydrides, forming hydrogenated compounds and iron(II) compounds:
Iron(II) compounds can also be prepared from an iron dihydride and an appropriate, concentrated acid:
Even though complexes containing dihydridoiron was known since 1931, [9] 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. 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. [9] The most precisely characterised FeH2L4 complex as of 2003 is FeH2(CO)2[P(OPh)3]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. [6]
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 Fe
2H
4, and that it reacts with atomic hydrogen to produce trihydridoiron (FeH
3). [7] However, it was later proven that the product of the reaction was likely to have been hydrido(dihydrogen)iron (FeH(H
2)). [6]
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula H2. It is colorless, odorless, tasteless, non-toxic, and highly combustible. Hydrogen is the most abundant chemical substance in the universe, constituting roughly 75% of all normal matter. Stars such as the Sun are mainly composed of hydrogen in the plasma state. Most of the hydrogen on Earth exists in molecular forms such as water and organic compounds. For the most common isotope of hydrogen (symbol 1H) each atom has one proton, one electron, and no neutrons.
In chemistry, there are three definitions in common use of the word base, known as Arrhenius bases, Brønsted bases, and Lewis bases. All definitions agree that bases are substances which react with acids as originally proposed by G.-F. Rouelle in the mid-18th century.
In chemistry, a hydride is formally the anion of hydrogen, H−. The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms are called hydrides: water (H2O) is a hydride of oxygen, ammonia is a hydride of nitrogen, etc. For inorganic chemists, hydrides refer to compounds and ions in which hydrogen is covalently attached to a less electronegative element. In such cases, the H centre has nucleophilic character, which contrasts with the protic character of acids. The hydride anion is very rarely observed.
Borderline hydrides typically refer to hydrides formed of hydrogen and elements of the periodic table in group 11 and group 12 and indium (In) and thallium (Tl). These compounds have properties intermediate between covalent hydrides and saline hydrides. Hydrides are chemical compounds that contain a metal and hydrogen acting as a negative ion.
Dihydrogen complexes are coordination complexes containing intact H2 as a ligand. They are a subset of sigma complexes. The prototypical complex is W(CO)3(PCy3)2(H2). This class of compounds represent intermediates in metal-catalyzed reactions involving hydrogen. Hundreds of dihydrogen complexes have been reported. Most examples are cationic transition metals complexes with octahedral geometry.
Atomic carbon, systematically named carbon and λ0-methane, also called monocarbon, is a colourless gaseous inorganic chemical with the chemical formula C. It is kinetically unstable at ambient temperature and pressure, being removed through autopolymerisation.
Positronium hydride, or hydrogen positride is an exotic molecule consisting of a hydrogen atom bound to an exotic atom of positronium. Its formula is PsH. It was predicted to exist in 1951 by A Ore, and subsequently studied theoretically, but was not observed until 1990. R. Pareja, R. Gonzalez from Madrid trapped positronium in hydrogen laden magnesia crystals. The trap was prepared by Yok Chen from the Oak Ridge National Laboratory. In this experiment the positrons were thermalized so that they were not traveling at high speed, and they then reacted with H− ions in the crystal. In 1992 it was created in an experiment done by David M. Schrader and F.M. Jacobsen and others at the Aarhus University in Denmark. The researchers made the positronium hydride molecules by firing intense bursts of positrons into methane, which has the highest density of hydrogen atoms. Upon slowing down, the positrons were captured by ordinary electrons to form positronium atoms which then reacted with hydrogen atoms from the methane.
Transition metal hydrides are chemical compounds containing a transition metal bonded to hydrogen. Most transition metals form hydride complexes and some are significant in various catalytic and synthetic reactions. The term "hydride" is used loosely: some so-called hydrides are acidic (e.g., H2Fe(CO)4), whereas some others are hydridic, having H−-like character (e.g., ZnH2).
Beryllium hydride is an inorganic compound with the chemical formula n. This alkaline earth hydride is a colourless solid that is insoluble in solvents that do not decompose it. Unlike the ionically bonded hydrides of the heavier Group 2 elements, beryllium hydride is covalently bonded.
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 °C.
Binary compounds of hydrogen are binary chemical compounds containing just hydrogen and one other chemical element. By convention all binary hydrogen compounds are called hydrides even when the hydrogen atom in it is not an anion. These hydrogen compounds can be grouped into several types.
Cadmium hydride is an inorganic compound with the chemical formula (CdH
2)
n. It is a solid, known only as a thermally unstable, insoluble white powder.
An iron hydride is a chemical system which contains iron and hydrogen in some associated form.
Mercury(II) hydride is an inorganic compound with the chemical formula HgH
2. It is both thermodynamically and kinetically unstable at ambient temperature, and as such, little is known about its bulk properties. However, it known as a white, crystalline solid, which is kinetically stable at temperatures below −125 °C (−193 °F), which was synthesised for the first time in 1951.
Copper hydride is inorganic compound with the chemical formula CuHn where n ~ 0.95. It is an red solid, rarely isolated as a pure composition, that decomposes to the elements. Copper hydride is mainly produced as a reducing agent in organic synthesis and as a precursor to various catalysts.
Chromium(I) hydride, systematically named chromium hydride, is an inorganic compound with the chemical formula (CrH)
n. It occurs naturally in some kinds of stars where it has been detected by its spectrum. However, molecular chromium(I) hydride with the formula CrH has been isolated in solid gas matrices. The molecular hydride is very reactive. As such the compound is not well characterised, although many of its properties have been calculated via computational chemistry.
Chromium(II) hydride, systematically named chromium dihydride and poly(dihydridochromium) is pale brown solid inorganic compound with the chemical formula (CrH
2)
n. Although it is thermodynamically unstable toward decomposition at ambient temperatures, it is kinetically metastable.
Iron(I) hydride, systematically named iron hydride and poly(hydridoiron) is a solid inorganic compound with the chemical formula (FeH)
n. It is both thermodynamically and kinetically unstable toward decomposition at ambient temperature, and as such, little is known about its bulk properties.
Chlorobis(dppe)iron hydride is a coordination complex with the formula HFeCl(dppe)2, where dppe is the bidentate ligand 1,2-bis(diphenylphosphino)ethane. It is a red-violet solid. The compound has attracted much attention as a precursor to dihydrogen complexes.
Hydrogen chalcogenides are binary compounds of hydrogen with chalcogen atoms. Water, the first chemical compound in this series, contains one oxygen atom and two hydrogen atoms, and is the most common compound on the Earth's surface.
The Journal of Chemical Physics, volume 104, issue 12, page 4859 ISSN 0021-9606 doi : 10.1063/1.471180 