The Coordination Chemistry and Electrochemistry of Chromium (III) Complexes of Thiobis (Ethylenenitrilo) Tetraacetic Acid

The product of the reaction between Cr(Cl04)3 and the ligand, thiobis(ethylenenitrilo)tetraacetic acid (TEDTA), was very dependent on the pH of the reaction mixture. Two monomeric isomers and several dimeric species were identified. Characterization of the various Cr(III) complexes of TEDTA included an examination of their pH behavior, visible and ultraviolet absorption spectra, substitutional !ability with excess azide ion, behavior on a Selectacel anion exchange column, reaction with heavy metal cations and methyl mercury cation, infra-red spectra and distinctive electrochemistry. Comparisons of the behavior of the Cr(III)-TEDTA complexes with those of the Cr(III) complexes of related ligands, including EDTA, oxybis(ethylenenitrilo)tetraacetic acid (EEDTA) and pentamethylenedinitrilotetraacetic acid (PMDTA), were very helpful. Based on these data, it was concluded that both cis and trans isomers (with respect to the nitrogens) were formed. In the trans isomer, the sulfur is forced to be close to the Cr(III) center and coordination occurs. At pH 5-9, solutions of the pentadentate cis isomer are a mixture of the monomeric complex and an acetate bridged dimer. Confirmation of dimer formation and a value for the dimerization constant were obtained from potential step experiments. The structure of the dimer was strongly supported by the cyclic voltammetry which indicated the presence of two nonequivalent metal centers in the complex. The other dimeric species were highly charged and their IR spectra suggested that they were oxo-bridged. The electrochemistry of both the cis and trans isomers and the acetate bridged dimer was investigated in detail. Cyclic voltammetry and electrocapillary measurements indicated that trans-CrTEDTA was much more strongly adsorbed on mercury electrodes than was cis-CrTEDTA(OH2), cis-CrTEDTA(OH2) or the acetate bridged dimer, and that free TEDTA was not adsorbed. Experiments with CrEEDTA showed that it was not adsorbed, indicating that the Cr(III)-TEDTA complexes were attached to the electrode through the sulfur. Quantitative measurements of the adsorption confirmed all of this. The adsorption of trans-CrTEDTA increased abruptly over a very narrow range of bulk concentrations and was strongest at positively charged electrodes. The potential dependence was corroborated by the fact that the charge on the electrode in the presence of the complex was more positive. The concentration and potential dependences are attributed to back bonding from Cr(III) to the thioether sulfur which a-donates electron density to the electrode. The abrupt change in the surface coverage over a narrow charge of bulk concentration indicates that the adsorption of trans-CrTEDTA is cooperative. Chronocoulometric measurements of the adsorption of the cis isomer at pH 4 and 7 demonstrated that the neutral complex was more strongly adsorbed than the anions especially at more negative potentials. The adsorption appeared to increase as the electrode potential was made more negative. However, the electrocapillary curves were discrepant. Electrode charge measurements supported the electrocapillary data. The adsorption of the cis complex depressed the double layer capacitance at all potentials. This behavior is similar to that of organic adsorbates. Although the non-adsorption of the free ligand was attributed to steric difficulty 1n exposing the sulfur atom to the electrode, the fact that the La(III) and Zn(II) complexes of TEDTA were also not adsorbed shows that the role of Cr(III) in inducing the adsorption of TEDTA in the cis isomers is complicated.