The complexation reactions of Mn2+, Co2+, Y3+, and ZrO2+ cations with the macrocyclic ligand, 4'- nitrobenz-15-crown-5 (4'-NB15C5), in acetonitrile (AN), methanol and AN–MeOH binary mixtures have been studied at various temperatures using the conductometric method. The conductance data stand for the Me : L stoichiometric ratio 1 : 1. Values of the formation constants of the complexes were accumulated by plotting molar conductivity curves using the computer program, GENPLOT. The order of stability of the metal–ion complexes in pure AN at 15◦C was found to be: (4'-NB15C5 · ZrO)2+ > (4'-NB15C5 · Mn)2+ ≈ (4'-NB15C5 · Co)2+ >(4'-NB15C5 · Y)3+ . In the case of AN–MeOH binary solvent solutions with 50 and 75 mole percent of AN at the same temperature, the sequence of stability of the complexes was the following: (4'-NB15C5 · Mn)2+ > (4'-NB15C5 · ZrO)2+ ≈ (4'-NB15C5 · Co)2+ > (4'-NB15C5 · Y)3+, and (4'-NB15C5 · Mn)2+ > (4'-NB15C5 · Y)3+ ≈ (4'-NB15C5 · Co)2+ > (4'-NB15C5 · ZrO)2+, respectively. The complexes formed are entropy stabilized in all cases.

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