Chromium(II) hydride, systematically named chromium dihydride and poly(dihydridochromium) is pale brown 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 or ). Although it is thermodynamically unstable toward decomposition at ambient temperatures, it is kinetically metastable. Chromium(II) hydride is the second simplest polymeric chromium hydride (after

chromium(I) hydride Chromium(I) hydride, systematically named chromium hydride, is an inorganic compound with the chemical formula (also written as or CrH). It occurs naturally in some kinds of stars where it has been detected by its spectrum. However, molecular ...

). In metallurgical chemistry, chromium(II) hydride is fundamental to certain forms of chromium-hydrogen alloys.

Nomenclature

The most common name for chromium(II) hydride is ''chromium dihydride'', following the

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 ...

compositional nomenclature. Because the compositional name does not distinguish between different compounds with stoichiometry , "chromium dihydride" is ambiguous between an unstable molecular species (see ) and the metastable (but common) polymeric form.

Monomer

The chromium(II) hydride monomer, is both thermodynamically and kinetically unstable towards autopolymerization at ambient temperature, and so cannot be concentrated. Nevertheless, molecules of and have been isolated in solid gas matrices. Cr is the second simplest molecular chromium hydride (after

Chromium(I) hydride Chromium(I) hydride, systematically named chromium hydride, is an inorganic compound with the chemical formula (also written as or CrH). It occurs naturally in some kinds of stars where it has been detected by its spectrum. However, molecular ...

). In the presence of pure hydrogen, dihydridochromium readily converts to bis(dihydrogen)dihydridochromium, CrH₂(H₂)₂ in an exothermic reaction.

Properties

Structure

In diluted CrH₂, the molecules are known to oligomerise forming at least Cr₂H₄ ( dimers), being connected 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. The dissociation

enthalpy Enthalpy , a property of a thermodynamic system, is the sum of the system's internal energy and the product of its pressure and volume. It is a state function used in many measurements in chemical, biological, and physical systems at a constant ...

of the dimer is estimated to be 121 kJ mol⁻¹. CrH₂ is bent, and is weakly repulsive to one hydrogen molecule, but attractive to two molecules of hydrogen. The bond angle is 118±5°. The stretching force constant is 1.64 mdyn/Å. The dimer has a distorted rhombus structure with ''C''_2''h'' symmetry.

Production

The dimer is produced synthetically by

hydrogenation Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a Catalysis, catalyst such as nickel, palladium or platinum. The process is commonly employed to redox, reduce or S ...

. In this process, chromium and hydrogen react according to the reaction: :Cr + H2 → HCr(μ-H)₂CrH This process involves atomic chromium as an intermediate, and occurs in two steps. The hydrogenation (step 2) is a spontaneous process. #Cr (s) → Cr (g) #Cr (g) + (g) → HCr(μ-H)₂CrH (g) In an inert gas matrix atomic Cr reacts with H₂ to make the dihydride when it is irradiated with

ultraviolet light Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...

between 320 and 380 nm. The reaction of chromium with molecular hydrogen is

endothermic In thermochemistry, an endothermic process () is any thermodynamic process with an increase in the enthalpy (or internal energy ) of the system.Oxtoby, D. W; Gillis, H.P., Butler, L. J. (2015).''Principle of Modern Chemistry'', Brooks Cole. p. ...

. 380 nm or greater wavelength radiation is required to procure photochemically generated CrH₂.

History

In 1979 the simplest molecular chromium(II) hydride with the chemical formula (systematically named dihydridochromium) was synthesised and identified for the first time. It was synthesised directly from the elements, in a reaction sequence which consisted of simultaneous

sublimation Sublimation or sublimate may refer to: * ''Sublimation'' (album), by Canvas Solaris, 2004 * Sublimation (phase transition), directly from the solid to the gas phase * Sublimation (psychology), a mature type of defense mechanism * Sublimate of mer ...

chromium Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal. Chromium metal is valued for its high corrosion resistance and hardne ...

to atomic chromium and

thermolysis Thermal decomposition, or thermolysis, is a chemical decomposition caused by heat. The decomposition temperature of a substance is the temperature at which the substance chemically decomposes. The reaction is usually endothermic as heat is req ...

hydrogen 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 . It is colorless, odorless, tasteless, non-toxic, an ...

, and concluded with co-deposition in a

cryogenic In physics, cryogenics is the production and behaviour of materials at very low temperatures. The 13th IIR International Congress of Refrigeration (held in Washington DC in 1971) endorsed a universal definition of “cryogenics” and “cr ...

argon Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...

matrix to form dihydridochromium. In 2003 the dimer with the chemical formula HCr(μ-H)₂CrH (systematically named ) was synthesised and identified for the first time. It was also synthesised directly from the elements, in a reaction sequence which consisted of

laser ablation Laser ablation or photoablation (also called laser blasting) is the process of removing material from a solid (or occasionally liquid) surface by irradiating it with a laser beam. At low laser flux, the material is heated by the absorbed laser ...

of chromium to atomic chromium, followed by co-deposition with hydrogen in a cryogenic matrix to produce dihydridochromium, and concluded with annealing to form .

References

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