Structure and dynamics of a compressed dihydride complex of osmium

The complex OsCl(NH=C(Ph) C6H4)(PiPr(3))(2)(H-2) ( 1) has been characterized by Esteruelas and coworkers (Organometallics 1998, 17, 4065-4076) as an elongated dihydrogen complex based on relaxation time measurements and observation of J(H-D) = 6.3 Hz in 1-d(1). Variable-temperature NMR studies were reported in which the single hydride resonance observed at ambient temperature decoalesced into two signals at low temperature ( 193 K), which was attributed to hindered rotation of the bound H-2 ligand. Reinvestigation of the low-temperature NMR spectra of 1 reveals a more complex situation. Examination of the H-1 and P-31 NMR spectra of 1 and 1-d(1) at lower temperatures than previously studied demonstrates that quantum mechanical exchange coupling is observed between the metal-bonded hydrogen atoms. This coupling is quenched in 1-d(1). Hindered rotation around Os-P bonds in 1 also contributes to the observed NMR spectra.