Rate constants for the formation and dissociation of a number of cobalamins, CBM-L (L = SCN-, SO32-, S2O32-, NCO-, N3-, I-, and Br-), have been determined using fast-reaction techniques. The formation rate constants are nearly the same order of magnitude (170-2300 M-1 sec-1), whereas the dissociation rate constants range between 590 and ≲ 10-5 sec-1 at 25°. The results are consistent with a
... [Show full abstract] transition state in which both entering and leaving groups are at most loosely bound to the cobalt atom. It was found that the large rate differences between aquocobalamin and the kinetically inert model cobalamin complexes, iodo- and nitroaquobis(dimethylglyoximato)cobalt(III), arise mostly from differences in activation entropies.