Protonation of Metal Hydrides by Strong Acids. Formation of an Equilibrium Mixture of Dihydride and Dihydrogen Complexes from Protonation of Cp*Os(CO)₂H. Structural Characterization of [CpW(CO)₂(PMe₃)(H)₂]⁺OTf⁻

Cp*Os(CO)₂H is protonated by triflic acid (HOTf) in CD₂Cl₂ solution to give an equilibrium mixture (87:13) of the dihydride [Cp*Os(CO)₂(H)₂]⁺OTf⁻ and the dihydrogen complex [Cp*Os(CO)₂(η²-H₂)]⁺OTf⁻. The acidity of these protonated species is roughly comparable to HOTf, since only partial protonation was observed, e.g., 36% protonation with 1.2 equiv of HOTf. In the absence of acid, the T₁ of the hydride ligand of Cp*Os(CO)₂H is 5.9 s at −80 °C. When all of the Cp*Os(CO)₂H is protonated by excess HOTf, the T₁ (−80 °C) of the terminal hydride ligands of [Cp*Os(CO)₂(H)₂]⁺OTf⁻ is 2.8 s, while the T₁ of the dihydrogen ligand of [Cp*Os(CO)₂(η²-H₂)]⁺OTf⁻ is 19 ms (−80 °C). The observed T₁ values of the Os−H resonance of Cp*Os(CO)₂H decreased significantly under conditions of partial protonation, indicating intermolecular proton transfer among [Cp*Os(CO)₂(η²-H₂)]⁺OTf⁻, [Cp*Os(CO)₂(H)₂]⁺OTf⁻, Cp*Os(CO)₂H, and HOTf. IR spectra indicate that the two CO ligands of [Cp*Os(CO)₂(H)₂]⁺ (and hence the hydrides as well) are trans to each other in the four-legged piano stool geometry. Two resonances for HOTf are observed in the NMR spectra and are assigned as [HOTf]_n (hydrogen bonded to itself) and TfOH···OTf⁻ in which HOTf is hydrogen bonded to an OTf⁻ counterion. [CpW(CO)₃(H)₂]⁺OTf⁻ and [Cp*W(CO)₃(H)₂]⁺OTf⁻ were formed by protonation of CpW(CO)₃H and Cp*W(CO)₃H. Protonation of the phosphine-substituted tungsten hydrides CpW(CO)₂(PR₃)H (R = Me, Cy, Ph) by HOTf or [H(Et₂O)₂]⁺ BAr‘₄⁻ (Ar‘ = 3,5-bis(trifluoromethyl)phenyl) gives dihydrides [CpW(CO)₂(PR₃)(H)₂]⁺ which were isolated and fully characterized. The structure of [CpW(CO)₂(PMe₃)(H)₂]⁺OTf⁻ was determined by single-crystal X-ray diffraction and reveals weak hydrogen bonds between one hydride on W and two of the fluorines on the triflate anion.