Chromium(II) hydride

Chromium(II) hydride, systematically named chromium dihydride and poly­(dihydridochromium) is pale brown solid inorganic compound with the chemical formula (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). In metallurgical chemistry, chromium(II) hydride is fundamental to certain forms of chromium-hydrogen alloys. 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 , "chromium dihydride" is ambiguous between an unstable molecular species (see ) and the metastable (but common) polymeric form. 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). In the presence of pure hydrogen, dihydridochromium readily converts to bis(dihydrogen)dihydridochromium, CrH2(H2)2 in an exothermic reaction. In diluted CrH2, the molecules are known to oligomerise forming at least Cr2H4 (dimers), being connected by covalent bonds. The dissociation enthalpy 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 C2h symmetry. The dimer is produced synthetically by hydrogenation. 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.