Kinetics of Formation and Dissociation of the Cryptates Ag(2,2,2)+ and K(2,2,2)+ in Acetonitrile+Water Mixtures at 25 deg C

Article abstract of DOI:10.1021/j100231a018

The dissociation rates of Ag(2,2,2)⁺ and K(2,2,2)⁺ in acetonitrile (AN)+ water system show a quite different dependence upon solvent mole fraction.The dissociation rate constant, k_d, of Ag(2,2,2)⁺ is almost indepependent of solvent composition and the rapid decrease of the stability constant, K_s, near x_AN = 0 is determined completely by the variation in the formation rate constant, k_f.The constant value for k_d for Ag(2,2,2)⁺ in the mixtures indicates that in the transition state the silver ion is strongly bonded to the (2,2,2) nitrogen atoms in a manner typical of the partially covalent interaction of monovalent d¹⁰ ions with nitrogen donors (e.g., in nitrilic solvents).This result, and the very similar variation of the Gibbs free energies of the transition state and stable cryptate complex with solvent composition, suggests that the transition state is very close to that of the products.The result is striking because for alkali-metal cryptates, particularly in nonaqueous solvents, a very similar solvent dependence is found for the reactants and transition state.The interaction between K⁺ and (2,2,2) is also found to be very different in this binary solvent system from that of Ag(2,2,2)⁺.Both k_d andK_f contribute similarly to the increase of the stability constant of K(2,2,2)⁺ with increasing mole fraction of acetonitrile, and comparisons of the Gibbs free energies of reactants, transition state, and product do not indicate any simple correlations between the solvation behavior of the three over the whole range of solvent composition.