A novel dinuclear calcium complex, [Ca-2{(2-OCO-3-CH3C6H3NHCO)(2)C(CH3)(2)}(2)(CH3OH)(6)] (1), was synthesized as a structural model of 8-coordinated Ca(II) ions in the double calcium-binding site of thermolysin. The complex has four NH ... O hydrogen bonds between the amide NH and the carboxylate oxygen anion. Two types of bridging coordination of the carboxylate ligand to Ca(II) were found in 1. The amide NH forms a strong NH ... O hydrogen bond with the anionic oxygen of the two carboxylate oxygens. A ligand-exchange reaction between the dinuclear calcium complex and eight equimolar amounts of 2,4,6-trimethylbenzoic acid or 2-CH3-6-t-BuCONHC6H3COOH indicates that the NH ... O hydrogen bond prevents the dissociation of the Ca-O bond.