Metal complexes of ethylenetetrathioglycollic acid and related ligands
Malcolm, Donna Evelyn
The purpose of this work has been primarily the investigation of the complexes formed by etnylenetetrathioglycollic acid with Co(II), Ni (II), Cu (II) and Zn (II). This ligand has some similarities to themuch-studied ethylenediaminetetraacetic aoid. The complexes of ethylenetetrathioglycollic acid have been studied as the solid complexes and as the complexes formed in aqueous solution. The solid complexes as prepared have metal to ligand ratio 2: 1 and contain bound water molecules, the number being variable but never less than two. The magnetic susoceptibilities and electronic spectra of the complexes of Co (II), Ni (rr) and Cu (II) have been found to be consistent with an octahedral arrangement of donor atoms about the central metal ion. The infrared spectra of the solid complexes indicated that the bonding of the carboxyl group is primarily ionic. Some preliminary studies have been done on the solid complexes of several other sulfidecarboxylic acids related to ethylenetetrathioglycollic acid.\ud \ud The nature of the oomplexes in solution has been studied through the electronic absorption spectra and through the measurement of stability constants. The spectra of the complexes of Ni (II) and cu (II) indicated that these complexes are octahedral. The absorption maxima occurred at slightly lower energies than those observed for the corresponding hexaquo ions, and it is suggested that ethylenetetrathioglycollic acid should be located just above water in the spectrochemical series. The spectrum of the complex of Co (II) has not been studied since the\ud complex was found to decompose in aqueous solution above pH 4. \ud Quantitative work has been started to determine the stability constants of the complexes formed in solution. As the first step in this direction, the acid dissociation constants of the ligand have been obtained. A least squares adjustment of potentiometric data derived from the usual pH- titration curves has given the following values: pK1a 2.89, pK2a 3.54, pK3a 3.958, and pK4a 4.572. The stability constants of the metal complexes have been sought using potentiometric and spectrophotometric techniques. A computer program has been written for the least squares adjustment of the potentiometric data, taking into account, in addition to the formation of a simple 1 : 1 complex, the formation of a binuclear complex and various protonated complexes. As yet this program has not given a reasonable set of stability constants. From the spectrophotometric measurements the value of log B11 for the nicke1 complex was found to be 4.17, and for the copper complex 5.72.\ud Stability constants were also found for the equilibria with the protonated ligand as in\ud \ud B + HA = BHA\ud \ud the value of log B111 being for nickel 2.9 and for copper 3.4. Evidence has also been found for the formation of at least one more complex, which is considerably less stable than the simple 1 : 1 complex and is probably a binuclear complex.\ud \ud In general, the studies on the nickel and the copper complexes of ethylenetetrathioglycollic acid have indicated that a group of closely related, weak complexes are formed. This is in contrast to ethylenediaminetetraacetic acid whioh forms very stable, simple 1 : 1 complexes with these metals.\ud \ud Most of the experimental data is compiled in Appendix A. The computer programs developed and used are given in Appendix B.
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