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The stability constants and rates of formation and dissociation of alkaline earth cryptates with (2,1,1), (2,2,1), (2,2,2), (2b,2,2), and (2B,2B,2) have been measured in several solvents. The stability constants, Ks, are considerably larger and display higher selectivity than those of the monocyclic crown and diaza crown ethers and anionic ionophores. Values of Ks vary by over 10 orders of magnitude in the different solvents, increasing in the order Me2SO < DMF = H20 < MeOH < PC. Ligand selectivity is based predominantly on variations in dissociation rates, both within the series of cryptands and in comparison to other macrocyclic ligands. Formation rates do not conform to the simple Eigen-Wilkins dissociative-interchange mechanism (Zd). The rates show no correlation with solvent exchange rates, and are sensitive to cation size, cation-solvent interactions, and ligand flexibility. In strongly solvating media such as Me2SO, rates are up to 106 lower than predicted by a simple Zd mechanism. The results suggest that the complexation reaction involves essentially stepwise replacement of solvent by ligand donor atoms, but that even for relatively flexible macrocyclic ligands compensation for loss in solvation by ligand binding energy in the transition state is not complete.Crystal and Molecular Structure of the Complex between jym-Dibenzo-14-crown-4-oxyacetate and Li+,[C20H21O7]~'Li+'7.5H2O