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) 2 H is protonated by triflic acid (HOTf) in CD 2 Cl 2 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 (η 2 -H 2 )] + 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 T 1 of the hydride ligand of Cp*Os(CO) 2 H is 5.9 s at −80 °C. When all of the Cp*Os(CO) 2 H is protonated by excess HOTf, the T 1 (−80 °C) of the terminal hydride ligands of [Cp*Os(CO) 2 ( H ) 2 ] + OTf − is 2.8 s, while the T 1 of the dihydrogen ligand of [Cp*Os(CO) 2 (η 2 - H 2 )] + OTf − is 19 ms (−80 °C). The observed T 1 values of the Os−H resonance of Cp*Os(CO) 2 H decreased significantly under conditions of partial protonation, indicating intermolecular proton transfer among [Cp*Os(CO) 2 (η 2 -H 2 )] + OTf −, [Cp*Os(CO) 2 (H) 2 ] + OTf −, Cp*Os(CO) 2 H, 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) 3 H and Cp*W(CO) 3 H. Protonation of the phosphine-substituted tungsten hydrides CpW(CO) 2 (PR 3 )H (R = Me, Cy, Ph) by HOTf or [H(Et 2 O) 2 ] + BAr‘ 4 − (Ar‘ = 3,5-bis(trifluoromethyl)phenyl) gives dihydrides [CpW(CO) 2 (PR 3 )(H) 2 ] + which were isolated and fully characterized. The structure of [CpW(CO) 2 (PMe 3 )(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.