Ligand substitution equilibria of different alkylcobalamins (RCbl, R = Me, CH2Br, CH2CF3, CHF2, CF3) with cyanide have been studied. It was found that CN- first substitutes the 5,6-dimethylbenzimidazole (Bzm) moiety in the alpha -position, followed by substitution of the alkyl group in the beta -position trans to Bzm. The formation constants K-CN for the 1:1 cyanide adducts (R(CN)Cbl) were found to be 0.38 +/- 0.03, 0.43 +/- 0.03, and 123 +/- 9 M-1 for R = Me, CH2Br, and CF3, respectively. In the case of R = CH2CF3, the 1:1 adduct decomposes in the dark with CN- to give (CN)(2)Cbl. The unfavorable formation constants for R = Me and CH2Br indicate the requirement of very high cyanide concentrations to produce the 1:1 complex, which cause the kinetics of the displacement of Bzm to be too fast to follow kinetically. The kinetics of the displacement of Bzm by CN- could be followed for R = CH2CF3 and CF3 to form CF3CH2(CN)Cbl and CF3(CN)Cbl, respectively, in the rate-determining step. Both reactions show saturation kinetics at high cyanide concentration, and the limiting rate constants are characterized by the activation parameters: R = CH2CF3, DeltaH(double dagger) = 71 +/- 1 kJ mol(-1), DeltaS(double dagger) = -25 +/- 4 J K-1 mol(-1), and DeltaV(double dagger) = +8.9 +/- 1.0 cm(3) mol(-1); R = CF3, DeltaH(double dagger) = 77 +/- 3 kJ mol(-1), DeltaS(double dagger) = +44 +/- 11 J K-1 mol(-1), and DeltaV(double dagger) = +14.8 +/- 0.8 cm(3) mol(-1), respectively. These parameters are interpreted in terms of an Id and D mechanism for R = CH2CF3 and CF3, respectively. The results of the study enable the formulation of a general mechanism that can account for the substitution behavior of all investigated alkylcobalamins including coenzyme B-12.