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 l igand of CpW(CO)(2)-(PMe3)H (3) with the rate at which one is transferred t o the metal. Qualitative evidence that protonation of the hydride ligand is faster is offered by observation of fast exchange between 3 and CF3SO3D an d 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 be tween 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 norma l modes of 3, 5, and their deuterated analogues via IR and Raman spectrosco py. 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 ra te constant for deprotonation of 4a by 4-tert-butyl-N,N-dimethylaniline (6) , combined with the pK(a) data, provide quantitative evidence that the kine tic site of protonation of 3 is the hydride ligand.