The quinquedentate macrocyclic ligand cyclo-6,6'-[1,9-(2,5,8-trithianonane)]-2,2'-bipyridine ([15]aneS(3)bpy = L), containing two pyridyl nitrogens and three thiaether sulfurs as donor atoms, has been synthesized and complexed with copper. The (CuL)-L-II/I redox potential, the stabilities of the oxidized and reduced complex, and the oxidation and reduction electron-transfer kinetics of the complex reacting with a series of six counter reagents have been studied in acetonitrile at 25 degrees C, mu = 0.10 M (NaClO4). The Marcus cross relationship has been applied to the rate constants obtained for the reactions with each of the six counter reagents to permit the evaluation of the electron self-exchange rate constant, k(11). The latter value has also been determined independently from NMR line-broadening experiments. The cumulative data are consistent with a value of k(11) = 1 x 10(5) M-1 s(-1), ranking this among the fastest-reacting Cu-II/I systems, on a par with the blue copper proteins known as cupredoxins. The resolved crystal structures show that the geometry of the (CuL)-L-II and (CuL)-L-I complexes are nearly identical, both exhibiting a five-coordinate square pyramidal geometry with the central sulfur donor atom occupying the apical site. The most notable geometric difference is a puckering of an ethylene bridge between two sulfur donor atoms in the (CuL)-L-I complex. Theoretical calculations suggest that the reorganizational energy is relatively small, with the transition-state geometry more closely approximating the geometry of the (CuL)-L-II ground state. The combination of a nearly constant geometry and a large self-exchange rate constant implies that this Cu-II/I redox system represents a true geometric "entatic state."