RAMAN SPECTRA OF ETHYLENEDIAMINETETRAACETIC ACID AND ITS METAL COMPLEXES

Raman spectra were measured of aqueous solutions of EDTA and its complexes with Be2+, Mg2+, Ca2+, Mn2+, Cu2+, Zn2+, Cd2+, Hg2+, Bi3+, and Pb2+. For the free ligand, spectra are reported at pH 4.5 and 11 in H2O and at pH 4.5 in D2O. Of special interest is the Raman band near 920 cm-1 which is diagnostic of protonation of the ligand. In H2O, protonation appears to involve the acetate groups, in addition to the nitrogen atom, as if an internal H-bonded structure is formed. However, in D2O the spectrum changes significantly, indicating strong solvent involvement. For the metal complexes, Raman lines characteristic of metal-nitrogen stretching frequencies are observed in the 400-500-cm-1 region with intensities indicating significant covalent bonding for all the metal ions studied except Mg2+ and Ca2+. The frequency of the metal-nitrogen stretching bands decreases with increasing radius of the metal ion, independent of the stability of the complex. In no case are there any Raman lines characteristic of covalent bonding of the acetate groups which apparently is mainly electrostatic. For the complexes of Cu2+ and Hg2+ the spectra show changes at both low and high pH consistent with a single protonation of the ligand in acid solution and coordination of an OH- in basic solution. For the Cu2+ complex at pH 12, a line at 430 cm-1 is observed which can be assigned to Cu-OH stretching. © 1968, American Chemical Society. All rights reserved.