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 H-1 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 CeW(CO)(2)(PMe3)-H-2(+) (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.