Novel benzoic acid ligands with bulky amide groups at the ortho position, 2,6-(MeCONH)(2)C6H3CO2H (1) and 2,6-(t-BuCONH)(2)C6H3CO2H (2), and their tris- and tetrakis(carboxylate) complexes with Ca(II) and Tb(III) ions, (NEt4)(2)-[Ca-II {O2C-2,6-(t-BuCONH)(2)C6H3}(4)] (4), [Tb{O2C-2,6-(t-BuNHCO)(2)C6H3}(3)(H2O)(3)}] (5), and (NMe)(4)(Tb{O2C-2,6-(t-BuNHCO)(2)C6H3}(4)(thf)] (6), were synthesized. The formation of the (NHO)-O-... hydrogen bonds between the amide NH and carboxylate for 2, (NEt4){2,6-(t-BuCONH)(2)C6H3CO2} (3), and 4 was determined by H-1 NMR spectroscopy in solution and in the solid state (CRAMPS, IR). The ligand exchange reactions were attempted between 4 and a large excess of 2,4,6- Me3C6H3CO2H in chloroform-d solution; however, exchange reaction did not take place, indicating that the Ca(II) ions bound strongly to the carboxylate in 4. The Ca(II) ion binding properties with the benzoate derivatives were also examined using Tb(III) ion as a fluorescence probe. These results indicate that the (NHO)-O-... hydrogen bonding between the amide NH and the oxygen atom of the carboxylate contributes to strong Ca(II) binding and prevents the dissociation of the calcium-carboxylate bond. The X-ray structural analyses of these complexes revealed that the (NHO)-O-... hydrogen-bonded carboxylate ligands prefer the chelate-type coordination and create a mononuclear {Ca(O2CR)(4)}(2-) or {Tb(O2CR)(4)}(-) core with anionic charge, which is known only in the active site of calcium-binding proteins.