The prepn. of redox-responsive metallomacrocyclic ionophore I, which is easily obtained by the reaction of [(cymene)RuCl2]2 with 3-hydroxy-2-pyridone in the presence of Cs2CO3 or K2CO3, is described. The binding const. of the LiCl and the NaCl with I is comparable to that of high-affinity ionophores such as cryptands, and has a nearly perfect selectivity for Na+ over K+. Preliminary results indicate that structurally related metallacrown ethers having different arene ligands (e.g., C6Me6, C3H3Et3) instead of cymene can be assembled using the strategy described here. Since the selectivity of the ionophore is detd. by these ligands, redox-responsive receptors with designed specificities and solubilities can be envisioned. The crystal structure of I and its complex with Li and Na is detd