NO donor ligands were prepared by the condensation of methoxy substituted salicylaldehyde with 5-methylfurfurylamine (I and 2) and 4'-aminobenzo-15-crown 5 (3-5). New crown ether ligands of Schiff base type (3-5) containing recognition sites for alkali metal and transition guest cations. Ni(II) complexes (1a-5a) have been synthesized with bidentate NO donor Schiff base ligands (1-5) with Ni(CH3COO)(2).center dot 4H(2)O. Monotopic crystalline 1:1 (Na+:ligand) sodium complexes (3b-5b) of the crown ether ligands were also prepared. Schiff bases (1-5) and complexes (1a-5a, 3b-5b) were characterized by elemental analyses, FT-IR, H-1-,C-13-NMR and mass spectroscopies. The crystal structures of 1, la and 2 were verified by X-ray diffraction analysis. The tautomeric equilibria (phenol-imine, O-H center dot center dot center dot N and ketoamine, O-H center dot center dot center dot N forms) have been systematically studied by using UV-vis spectrophotometry for the ohydroxy substituted compounds (1-5). The UV-visible spectra of these ligands (1-5) were recorded and commented in polar, non-polar, acidic and basic media. (C) 2015 Elsevier B.V. All rights reserved.