Structure of the Oxygen-carrying Cobalt (II) Histidine Complex

Abstract

Bis-histidinato cobalt (II) is one of the few synthetic compounds which can combine reversibly with molecular oxygen. Hearon et al.1 obtained a value of 4.44 Bohr magnetons (B.M.) for its effective magnetic moment at room temperature in aqueous solution, and showed that on oxygenation it became almost diamagnetic with the absorption of one oxygen molecule for two cobalt atoms. For three unpaired electrons the spin-only moment would be 3.88 B.M., and the considerably larger experimental value obtained in this case was ascribed to incomplete quenching of the orbital contribution to the moment. From the magnetic measurements Hearon assigned a tetrahedral structure (sp3 hybridization), I, to the complex: in which the cobalt atom was co-ordinated to the α-amino group and an imidazole nitrogen atom of each histidine molecule. However, histidine can behave as a tridentate ligand, and an octahedral configuration, II, involving co-ordination of ionized carboxyl groups is not unlikely. Hearon considered this to be only a secondary possibility.