The most important, and almost unique, property of the macrocyclic polyethers ("crown compounds") is their tendency to form complexes with alkali metal salts and salts with similar cations. Such complexes are held together by electrostatic attraction between the cation and the negatice end of the C -0 dipoles. The stability of the polyether complexes depends primarily upon how well the cation fits into the polyether ring; other factors are the charge density ofthe cation andin solution-the solvating power of the medium. Cyclic polyethers have been successfully employed, inter alia, in experiments with ionic compounds in organic solvents and in studies of ion transport in biological systems.

The stability constants for the 1:1 complexes of 22 cyclic polyethers (12-to 60-membered rings of C-C-O units with various substituents including nitrogen and sulfur) with several cations (Li+, Na+, K+, Cs+, NH4+, Ag+) in water and in methanol have been determined by potentiometry with cation-selective electrodes. The methanol stability constants are three to four decades higher than the aqueous ones, presumably because water competes more strongly for the cation than does methanol. Selectivity toward the different cations varies with polyether ring size, the optimum ring size being such that the cation just fits into the hole, i.e., 15-18 for Na+, 18 for K+, and 18-21 for Cs+. The effects of side rings fused onto the main polyether ring are comparatively small and difficult to predict. Substitution of N or S for O reduces affinity for alkali ions but greatly strengthens complexing of Ag+. Evidence of 2:1 polyether:cation complexes was found for several systems in which the cation was larger than the hole of the ring. These results are consistent with Pedersen's conclusion that the alkali cation is held in the hole of the polyether ring by ion-dipole forces, though covalent bonding plays a part in the Ag+ complexes of polyethers containing N or S.(4) (a) .

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Die wichtigste und nahezu einzigartige Eigenschaft der makrocyclischen Polyäther („Kronenverbindungen”︁) ist ihre Tendenz zur Komplexbildung mit Alkalimetallsalzen und Salzen mit ähnlichen Kationen. Derartige Komplexe werden durch nichtgerichtete Coulomb‐Kräfte zwischen dem Kation und den negativen Enden der C—O‐Dipole zusammengehalten. Die Stabilität der Polyäther‐Komplexe hängt vor allem davon ab, wie gut das Kation in den Polyätherring hineinpaßt, aber auch von der Ladungsdichte des Kations und — in Lösung — von der Solvatationsstärke des Mediums. Die cyclischen Polyäther sind mit Erfolg u. a. bei Arbeiten mit Ionenverbindungen in organischen Lösungsmitteln sowie zur Untersuchung des Ionentransports in biologischen Systemen verwendet worden.