Molecular Hydrogen Complexes of the Transition Metals. 4. Preparation and Characterization of M(CO)3(PR3)2(η2-H2)(M = Mo, W) and Evidence for Equilibrium Dissociation of the H-H bond To Give MH2(CO)3(PR3)2

Article abstract of DOI:10.1021/ja00282a026

The syntheses, properties, and spectral characterization of the first examples of molecular hydrogen complexes, M(CO)3(PR3)2(H2) (M = Mo, W; R3 = Cy3, i-Pr3, Cy2-i-Pr), are reported in full.All six of the expected fundamental vibrational modes for η²-H2 binding, including ν(HH) at 2690 cm^-1, have been located.The hydrogen atoms of the H2 ligand, but not of the phosphines, undergo exchange with D2 to give HD, even in the solid state.Solid-state 2H NMR of W(CO)3(P-i-Pr3)2(D2) shows rapid rotation of the D2 about the metal-D2 axis.IR and variable-temperature 1H and 31P NMR of solutions of the H2 complexes reveal the presence of equilibrium amounts (10-30percent) of a species that the data indicate is a 7-coordinate dihydride, MH2(CO)3(PR3)2.The latter is presumably formed by dissociation of the H-H bond, thus completing oxidative addition of H2 to the metal.The dihydride is fluctional, but low-temperature NMR spectroscopy shows that both the hydride and phosphorus ligands are inequivalent.At -80 deg C the T₁ value for the 1H NMR signal of the H2 ligand is 0.004 s, almost three orders of magnitude less than that of the hydride protons (1.7 s) in WH2(CO)3(P-i-Pr3)2.