Equilibrium constants (log K) for the substitution of coordinated H2O in aquacyanocobyrinic acid heptamethyl ester (aquacyanocobester, ACCbs) and stable yellow aquacyanocobyrinic acid heptamethyl ester (stable yellow aquacyanocobester, ACSYCbs), in which oxidation of the CS carbon of the corrin interrupts the normal delocalized system of corrins, by ligands with soft (CN-, SO32-, and s(2)O(3)(2-)) and hard (NO2- and N-3(-)) donors have been determined. The ligands with a harder donor atom (N in N-3(-) and NO2-) produce AH values that are more negative in their reactions with ACSYCbs than with ACCbs. If the donor atom is softer (C in CN- and S in SO32-), then Delta H is less positive, or more negative, for reactions with ACCbs than with ACSYCbs. The softer metal in ACCbs has a preference for softer ligands and the harder metal in ACSYCbs for the harder ligands. A kinetics study in which CN-substitutes H2O on Co-III shows that ACCbs is more labile than ACSYCbs; the second-order rate constant k(II) is between 4.6 (at 5 degrees C) and 2.6 (at 35 degrees C) times larger. Delta H-double dagger for the reaction of CN- with ACCbs is smaller by some 12 kJ moL(-1) than that for the reaction with ACSYCbs, consistent with an earlier transition state in which bonding between the softer metal of ACCbs and the ligand is greater than that of ACSYCbs with its harder metal. This difference in Delta H-double dagger makes ACCbs over 100 times more labile, although the effect is masked by a Delta s(double dagger) value that is over 30J K-1 mol(-1) more negative. There is a significant increase in the inertness of Co-III upon a decrease in the extent of conjugation of the corrin ligand. Modifying the electronic structure of the equatorial ligand in the cobalt corrins can modify the thermodynamics and kinetics of its reactions with exogenous ligands.