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Measurements have been made of the stability constants of complexes of the anionic ionophores monensin and lasalocid with alkali-metal cations, alkaline-earth-metal cations and Ag+ in several solvents, both protic and polar aprotic. The complexes of monensin are very stable and show a sharp stability maximum among the alkali-metal cations for the Na+ complex in all solvents. Compared with complexes of various neutral ionophores, the stability constants of monensin complexes are relatively much higher in aprotic than protic solvents. This may be attributed to a loss of solvation altering the free energy and conformation of the monensin anion on transfer from protic to aprotic solvents. Complexes of the smaller lasalocid ion are less stable, less sensitive to solvent variation and show a different selectivity pattern to those of monensin. Structural features of the two ligands responsible for the different complexing properties are discussed.

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