The Raman spectra of iron(III) complexes with ethylenediaminetetraacetate were observed for the purpose of elucidating the structure of the complexes in solution. In a Raman spectral study on the solid complexes, the spectra in the 300–600 cm−1 region were found to be diagnostic to coordination geometry. Namely, the coordination number can be determined on the basis of the wavenumber difference of the two major bands observed in the 450–600 cm−1 region; the wavenumber differences observed for heptacoordinate complexes lie between 32 and 42 cm−1, while that for the hexacoordinate complex is 62 cm−1. Complexes containing a sexidentate edta exhibit a single intense band around 470 cm−1 in the 450–500 cm−1 region, while complexes containing a quinquedentate edta exhibit two intense bands in this region. These Raman spectral criteria were then applied for the determination of the solution structure. The Raman spectral features observed for solutions of Fe(III)-edta complexes indicate that the any alkali metal salts and acid salts of FeIII(edta)− give sexidentate heptacoordinate complex ions in water. The dimeric complex which is formed in an alkaline solution was characterized as being a dinuclear complex consisting of two quinquedentate hexacoordinate iron(III) moieties with a Fe–O–Fe bridging unit.

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