Protonation of CpW(CO)2(PMe3)H:  Is the Metal or the Hydride the Kinetic Site?

We have compared the rate at which a proton is transferred to the hydride ligand of CpW(CO)2(PMe3)H (3) with the rate at which one is transferred to the metal. Qualitative evidence that protonation of the hydride ligand is faster is offered by observation of fast exchange between 3 and CF3SO3D and selective broadening of the trans hydride resonance of 3 during exchange with anilinium. Rate constants were obtained by 1H NMR for H/D exchange between 3 and 4-tert-butyl-N,N-dimethylanilinium-N-d 1 (5-d 1); an unusual EIE of 0.19 was observed and explained by location of the vibrational normal modes of 3, 5, and their deuterated analogues via IR and Raman spectroscopy. A pK a of 5.6(1) in CH3CN was determined for CpW(CO)2(PMe3)H2 + (4a) by IR; the importance of homoconjugate pair interactions in protonation equilibria is illustrated and discussed. The exchange rate data and the rate constant for deprotonation of 4a by 4-tert-butyl-N,N-dimethylaniline (6), combined with the pK a data, provide quantitative evidence that the kinetic site of protonation of 3 is the hydride ligand.