The (d(xy))(2) --> (d(xy))(1)(d(xz,yz))(1) excited state of trans-ReO2(py)(4)(+) is quenched by proton transfer from Tp’Mo-(CO)(3)H (Tp’ = hydridotris[(3,5-dimethyl)pyrazolyl]borate) and CpM(CO)(3)H (M = Cr, Mo, W). The pK(a)’s and proton self-exchange rate constants (k(22)) of these metal hydrides are well characterized in acetonitrile, The rate constants (k(Q)) for quenching of ReO2(py)(4)(+)* by these complexes were determined by Stern-Volmer quenching. The relative rate constants for the four metal hydrides are described by the Marcus cross relation, which predicts that a plot of log(k(Q)/k(22)(1/2)) versus pK(a) will be linear with a slope of 1/2. Ln fact, a linear correlation is observed with a slept of 0.53. From this relationship, values for k(22) can be calculated for metal hydrides where k(Q) and pK(a) are known. This method has been applied to TpMo(CO)(3)H and Mn(CO)(5)H (Tp = hydridotris(pyrazolyl)borate), which give k(22)’s of 200 and 1200 M(-1) s(-1), respectively. These rate constants are consistent with the steric and electronic factors that can be expected to govern the kinetic acidities. The isotope effects for quenching by the CpM(CO)(3)H complexes are the same within experimental error for all three complexes, as predicted by the Marcus cross relation and the known isotope effects on M-H self exchange. These data give an isotope effect for ReO(OH)(py)(4)(+)* of 1.2 +/- 0.1, which is consistent with the fast (10(7) M(-1) s(-1)) self-exchange estimated by other methods.