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Transition metal complexes of α-aminothioacids
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|Title:||Transition metal complexes of α-aminothioacids|
|Authors:||O'Halloran, Harry John|
|Abstract:||Metal complexes of several α-aminothioacids have been prepared and characterized. The ligands employed in this study were the thioacid analogs of α-aminoacids and included thioglycine (tg). DL-thiovaline (tv). DL-β-phenylthioalanine (pta). DL-thiomethionine (tm), and thioisoleucine (ti) (a mixture of isomers). The ligands' general structure is that of the zwitter ion RCH(NH3)COSˉ. Red, sparingly soluble nickel( II) complexes were prepared from each of the ligands. and in all cases only one product was obtained regardless of the stoichiometric amounts of reactants involved. The elemental analysis indicate that the complexes are the one metal to two ligand species, bis (thioglycinato) nickel(II) [Ni(tg)2], bis(thiovalinato)nickel(II) [Ni(tv)2], bis(β-phenylthioalaninato)nickel(II) [Ni(pta)2], bis(thiomethioninato)nickel( II) [Ni(tm)2], and bis(thioisoleucinato)nickel(II) [Ni(ti)2]. The low conductivity of these compounds indicate a nonelectrolyte, molecular structure. The magnetic moments and the electronic spectra corresponds to the supposition that the donor atoms are arranged in a square planar configuration surrounding the nickel ions. The complexes can exist in either the cis (I) or trans (II) form. Unfortunately, the very slight solubility of these 'complexes rendered the methods usually employed to distinguish between m and trans configurations experimentally infeasible. However, an x-ray analysis is presently in progress in these laboratories to determine the absolute structure of the nickel-thioglycine complex. Attempts to S-alkylate the ligands as either the free acids or while coordinated to the nickel atom failed. Also, efforts to form thiobridged complexes were unsuccessful. The reaction of cobalt(II) acetate with thioglycine produced an extremely insoluble, dark maroom complex of the composition Co(tg)2. The magnetic measurements suggest that the compound may be antiferromagnetic, indicating some sort of metal-metal orbital interaction, perhaps by a Co-S-Co delocalization or a direct Co-Co bond. Efforts to obtain cobalt complexes with the remaining ligands were unsuccessful. The metal ions Pt(II), Pd(II) and Cu(II) caused decomposition of the ligands. The reaction of the ligands with copper(II) bromide generated colorless solutions which indicated the formation of Cu(I) as an intermediate in the decomposition of the ligands. A potentiometric study over a range of temperatures was undertaken to determine the dissociation constants of thioglycine and the stability constants of the nickel(II)-thioglycine complex, as well as the thermodynamic values of these reactions. This system was then compared with the nickel(II)-glycine system. The Ka for thioglycine is much greater than the Ka for glycine. Also, Kb is larger for thioglycine than for glycine. Regardless of these facts, the nickel-thioglycine system forms a more stable complex than the corresponding nickel-glycine system. The K2 for the nickel-thioglycine system is actually larger than the K1. The thermodynamic data indicate that this phenomenon is an entropy effect probably caused by the change in configuration from an octahedral environment to a square planar one for the nickel ion.|
Thesis (Ph. D.)--University of Hawaii, 1969.
Bibliography: leaves 129-137.
ix, 137 l illus
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|Appears in Collections:||Ph.D. - Chemistry|
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