Iron(II) hydride

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]

Nomenclature

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

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

Dihydridoiron is a related compound with the chemical formula FeH4•
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

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

Radicality

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 (X⁵Δ_g, A⁵Π_g, B⁵Σ_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 predicts that the low transition energies correspond to a colourless compound.

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 ⁵Δ_g.^[2]

Chemical reactions

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

Production

Dihydridoiron 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.^[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]

Related

Although iron(II) hydride has received attention only recently, complexes containing the moiety have been known at least since 1931, when iron carbonyl hydride FeH₂(CO)₄ was first synthesised.^[3] 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.^[5]

References