Abstract
The structures of the complexes of Fe2+ and Mn2+ with citric acid were investigated by infrared spectroscopy. The compounds used were [Mn (H2O)6] [Mn (C6H5O7) H2O]2·2H2O, Mn (C6H6O7)·H2O and Na4 [Fe (C6H5O7)2]. In the IR spectra of the last complex, the strength of the band assignable to the stretching vibration of the alcoholic OH group of the citrate ion was weaker than in the free molecule. This result suggests that the OH group contributes to the chelate formation. Na4 [Fe (C6H5O7)2] is considered to be octahedral, with the two trivalent citrate anions binding to the central iron (II) ion. Thus, two of the three carboxyl groups and one OH group within a citrate ion combine with the Fe2+ ion, and one carboxyl group of the citrate ion is free. On the other hand, the Mn2+ ion within Mn (C6H6O7)·H2O crystals is considered to take a tetrahedral configuration, and again one of the three carboxyl groups of the coordinated citrate ion remains as nonbonded and undissociated COOH. In Mn (C6H6O7)·H2O, the water molecule seems to bind to the central metal ion. The carboxyl groups of [Mn (H2O)6] [Mn (C6H5O7)·H2O]2·2H2O compound acted as either monodentate or bridging ligand groups.
