C(2)-H isotopic exchange in coordinated imidazoles revisited.: The case of the [Co(NH3)5ImH]3+ ion

The temperature dependence of the acid ionization constants of [Co(NH3)(5)ImH](3+) in H2O (I = 1.0 M (NaClO4)): pK(a) (degrees C)= 10.10 +/- 0.04 (25.0), 9.92 +/- 0.03 (30.0), 9.82 +/- 0.02 (35.0), 9.62 +/- 0.03 (40.0), and [Co(ND3)(5)ImD](3+) in D2O (I = 0.35 M (NaClO4)): pK(a) (degrees C) = 10.58 +/- 0.06 (25.0), 9.46 +/- 0.08 (60.0) is reported. Observed first-order rate constants for H/D exchange at C-2 in [Co(ND3)(5)ImD](3+) over the pD range 8.08-11.20 (60.0 degrees C, I = 0.35 M (NaClO4)) follow an equation of the form: k(obs) = k(OD)K(w)/(a(D+) + K-a)gamma(+/-), with k(OD) (0.27 +/- 0.06 M-1 s(-1)) corresponding to the rate constant for OD--catalyzed abstraction of H-2 in [Co(ND3)(5)ImD](3+) and K-a ((2.8 +/- 0.7) x 10(-10) M, pK(a) = 9.55 +/- 0.13) to the acid ionization constant of this species. No evidence was found for a pathway to H/D exchange in the imidazolate moiety of [Co(ND3)(5)Im](2+).