Chemistry:Chromium(II) hydride

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Chromium dihydride
Names
Other names
Chromium dihydride
Identifiers
3D model (JSmol)
ChemSpider
Properties
CrH2
Molar mass 54.0040 g/mol
Appearance brown solid
Related compounds
Related compounds
chromium(I) hydride Chromium hydride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Chromium(II) hydride, systematically named chromium dihydride and poly­(dihydridochromium) is pale brown solid inorganic compound with the chemical formula (CrH2)n (also written ([CrH2])n or CrH2). 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). 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 compositional nomen­clature. Because the compositional name does not distinguish between different compounds with stoichiometry CrH2, "chromium dihydride" is ambiguous between an unstable molecular species (see § Monomer) and the metastable (but common) polymeric form. When disambiguation is necessary, the IUPAC also recognizes the additive name poly­(dihydridochromium) or the electron-deficient substitutive name poly[chromane(2)] for the polymer.

Monomer

The chromium(II) hydride monomer, also systematically named chromane(2), is both thermodynamically and kinetically unstable towards autompolymerization at ambient temperature, and so cannot be concentrated. Nevertheless, molecules of CrH2 and Cr2H4 have been isolated in solid gas matrices.[1]

Chromane(2) is the second simplest molecular chromium hydride (after chromane(1)). In the presence of pure hydrogen, dihydridochromium readily converts to bis(dihydrogen)dihydridochromium, CrH2(H2)2 in an exothermic reaction.[1]

Properties

Structure

In diluted chromane(2), the molecules are known to oligomerise forming at least dichromane(4) (dimers), being connected by covalent bonds. The dissociation enthalpy of the dimer is estimated to be 121 kJ mol−1.[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°.[2] The stretching force constant is 1.64 mdyn/Å.[2] The dimer has a distorted rhombus structure with C2h symmetry.

Production

Dichromane(4) is produced synthetically by hydrogenation. In this process, chromium and hydrogen react according to the reaction:

Cr + H2HCr(μ-H)2CrH[1]

This process involves atomic chromium as an intermediate, and occurs in two steps. The hydrogenation (step 2) is a spontaneous process.

  1. Cr (s) → Cr (g)
  2. Cr (g) + H 2(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 between 320 and 380 nm.[2] The reaction of chromium with molecular hydrogen is endothermic. 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 CrH2 (systematically named chromane(2) and 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 of chromium to atomic chromium and thermolysis of hydrogen, and concluded with co-deposition in a cryogenic argon matrix to form chromane(2).[3]

In 2003 the dimer with the chemical formula HCr(μ-H)2CrH (systematically named dichromane(4) and di-μ-hydrido-bis(hydridochromium)) 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 of chromium to atomic chromium, followed by co-deposition with hydrogen in a cryogenic matrix to produce chromane(2), and concluded with annealing to form dichromane(4).[1]

References

  1. 1.0 1.1 1.2 1.3 1.4 Wang, Xuefeng; Andrews, Lester (30 January 2003). "Chromium hydrides and dihydrogen complexes in solid neon, argon, and hydrogen: Matrix infrared spectra and quantum chemical calculations". The Journal of Physical Chemistry A 107 (4): 570–578. doi:10.1021/jp026930h. Bibcode2003JPCA..107..570W. 
  2. 2.0 2.1 2.2 Xiao, Z. L.; Hauge, R. H.; Margrave, J. L. (January 1992). "Reactions and photochemistry of chromium and molybdenum with molecular hydrogen at 12 K". The Journal of Physical Chemistry 96 (2): 636–644. doi:10.1021/j100181a024. 
  3. van Zee, R. J.; de Vore, T. C.; Welner Jr., W. (1 September 1979). "CrH and CrH2 molecules: ESR and optical spectroscopy at 4 °K [sic]". The Journal of Chemical Physics 71 (5): 2051–2056. doi:10.1063/1.438596. Bibcode1979JChPh..71.2051V.