Synthesis and Characterization of OsH₂Cl[κN,κO-(ONCR₂)](PⁱPr₃)₂ (CR₂ = C(CH₂)₄CH₂, R = CH₃):  Influence of the L₂ Ligand on the Nature of the H₂ Unit in OsH₂ClL₂(PⁱPr₃)₂ (L₂ = ONCR₂, NHC(Ph)C₆H₄) Complexes

The dihydride−dichloro complex OsH₂Cl₂(PⁱPr₃)₂ (1) reacts with cyclohexanone oxime and acetone oxime in the presence of Et₃N to give the dihydride derivatives (2) and OsH₂Cl{κN,κO-[ONC(CH₃)₂]}(PⁱPr₃)₂ (3), respectively. The structure of 2 has been determined by X-ray diffraction. The geometry around the osmium atom can be described as a distorted pentagonal bipyramid, with the triisopropylphosphine ligands occupying two relative trans positions. The remaining perpendicular plane is formed by the hydride ligands, the chlorine, and the oximate group, which acts with a bite angle of 36.6(1)°. In solution the hydride ligands of 2 and 3 undergo an intramolecular thermally activated site exchange process. The activation parameters of this process are ΔH^⧧ = 11.9(±0.7) kcal mol^-1 and ΔS^⧧ = −0.5(±1.4) cal mol^-1 K^-1 for 2 and ΔH^⧧ = 11.7(±0.8) kcal mol^-1 and ΔS^⧧ = −0.8(±2.0) cal mol^-1 K^-1 for 3. To understand why complexes 2 and 3 are dihyride derivatives, while the previously reported complex is an elongated dihydrogen compound, a quantitative theoretical analysis of the interaction between the H₂ moiety and the OsClL₂(PH₃)₂ (L₂ = ONCH₂, NHCHCHCH) complex fragments, along the oxidative addition pathway, is also reported.