Structure and Thermal Reactions of Dihydrogen Complexes: The IR Characterization of M(CO)5(H2) (M = Cr, Mo, and W) and cis-Cr(CO)4(H2)2 in Liquid Xenon Solution and the Formation of HD during Exchange of H2 and D2

Article abstract of DOI:10.1021/ja00273a017

UV photolysis of M(CO)6 (M= Cr, Mo, and W) and H2 in liquid Xe at -70 deg C yields M(CO)5(H2) and, for Cr and W, cis-M(CO)4(H2)2.For all three metals, M(CO)5(H2) has been fully characterized from IR spectra, including 13CO data, C-O factored force field analysis of ν(H-H) bands.Surplisingly, the ν(H-H) band for Mo, 3080 cm^-1, is at a higher wavenumber than Cr, 3030 cm^-1, or W, 2711 cm^-1.The thermal stability of the three compounds is in the order Mo(CO)5(H2) (least stable) << Cr(CO)5(H2) < W(CO)5(H2) (most stable).IR data for Cr(CO)5(D2) and Cr(CO)5(HD) are also presented.Similary, cis-Cr(CO)4(H2)2 has been characterized by 13CO enrichment.Cr(CO)5(H2) undergoes an exchange reaction, at -70 deg C, with N2 to form Cr(CO)5(N2), and cis-Cr(CO)4(H2)2 forms exclusively cis-Cr(CO)4(N2)2 under similar conditions via a detectable intermediate, probably cis-Cr(CO)4(N2).Under a pressure of D2, Cr(CO)5(H2) reacts thermally to form Cr(CO)5(D2) but not Cr(CO)5(HD).However, Cr(CO)5(HD) is formed during H2/D2 exchange in Cr(CO)4(H2)2.The HD is probably formed by intramolecular exchange in Cr(CO)4(H2)(D2) <*> Cr(CO)4(HD)2.By contrast, reaction of d⁸ metal centers with H2 in Xe solution does lead to formation of dihydrides.UV photolysis of Ru(CO)4PPh3 and H2 dissolved in supercritical Xe at room temperature yields Ru(CO)3(H2)PPh3, an unstable dihydride, previously reported in thermal reactions at higher temperature.