Available experimental data on stability constants of proton (hydron) and metal complexes for seven complexones of particular biomedical and environmental interest: iminodiacetic acid [2,2'-azanediyldiacetic acid, IDA], (methylimino)diacetic acid [2,2'-(methylazanediyl)diacetic acid, MIDA]; 2,2',2'',2'''-{[(carboxymethyl)azanediyl]bis[(ethane-1,2-diyl)nitrilo]}tetraacetic acid (DTPA), 3,6,9,12-tetrakis(carboxymethyl)-3,6,9,12-tetraazatetradecanedioic acid (TTHA); 2,2',2''-(1,4,7-triazonane-1,4,7-triyl)triacetic acid (NOTA); 2,2',2'',2'''-(1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl)tetraacetic acid (DOTA); 2,2',2'',2'''-(1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetrayl)tetraacetic acid (TETA), published in 1945-2000, have been critically evaluated. Some typical errors in stability constant measurements for particular complexones are summarized. Higher quality data are selected and presented as “Recommended” or “Provisional”.
Pyridine Derivatives as Complexing Agents XI. Thermodynamics of Metal Complex Formationwith Bis-, Tris-and TetrakisI(2-pyridyl)methyl]-amines. SummaryThe equilibria between H+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Cd2+, Pb2+, Hg2+ and Ag+, and the ligands bis(2-pyridylmethyl)-amine ( = DPA), tris(2-pyridylmethy1)-amine (= TPA), tris(6-methyl-2-pyridylmethyl)-amine (= TLA) and N, N, N', N'-tetrakis(2-pyridylmethyl)-ethylenediamine ( = TPEN) have been studied. Only the stability constants of DPA and TLA with almost all these cations were obtained using the pH method. For the other ligands, the complexes are already formed in acid solutions and only the use of different ligand-ligand or metal-metal exchanges as well as of pM methods were successful.The protonation constants indicate that for DPA the protonation occurs firstly at the aliphatic nitrogen atom whereas in all other cases only the pyridine groups can be protonated. The thermodynamic functions of protonation are in agreement with this interpretation.The stability constants of the complexes are often similar in magnitude to those of the analogous aliphatic amines, in spite of the much lower basicities of the pyridine derivatives. The Fe(l1)Ns species of DPA and TPEN are appreciably more stable than those of the corresponding aliphatic ligands. This is due to the formation of low-spin complexes with an unexpected A H value. Comparison of the thermodynamic data of formation of the complexes with TPA and TLA shows the effect of the three bulky methyl groups of the second ligand. As a consequence of steric hindrance and of the major dehydration, A H and less A S are more positive for M(TLA)2+ than for M(TPA)2+. Therefore M(TLA)2+ is normally much less stable than M(TPA)2+ The data for MnTPA2+ and ZnTPA2+ appear to indicate that in these complexes the coordination number of the metal ion is seven and four respectively. In addition to the complexes ML2+, with these two ligands hydroxo complexes ML(OH)+ are formed at remarkably low pH. Further TPEN seems to be sexidentate in the 1 : 1 complexes with Mn2+, Co2+ and Ni2+ but quinquedentate in those with Cu2+ and Zn2+, also in agreement with the spectra in solution and of the solid complex salts. The reaction: M(DPA)z2++TPEN + M(TPEN)2++2DPA is for all metal ions favoured by A H and AS, whereas in the case of the corresponding aliphatic ligands only by the second term. This result is explained in terms of a different magnitude of hydration of the two sexidentate ligands as a consequence of the presence of the hydrophobic aromatic rings in TPEN. I )X. Teil vgl. [l].
The equilibria between acethydroxamic acid and 16 metal cations, and their complexes have been elucidated. For the results see table 1. The complex Formation of desferri‐ferrioxamine B and desferri‐ferrioxamin E, which are trihydroxamic acids, with various bivalent and trivalent metal ions have been studied in a similar manner. For the results see tables 2 and 3.
Within the scope of the OECD Nuclear Energy Agency (NEA) Thermochemical Data Base Project (TDB) a comprehensive review of selected organic ligands has been carried out by the authors. The selected ligands are oxalate, citrate, ethylenediaminetetraacetate (edta) and α-isosaccharinate (isa), and the elements considered in the review are U, Np, Pu, Am, Tc, Ni, Se and Zr, as well as the necessary basic data concerning protonation of the ligands and interactions with the major competing elements Na, K, Mg and Ca. This review on organic ligands showed that the pragmatic ionic strength correction procedure, the Specific ion Interaction Theory (SIT), chosen as the default method for all NEA TDB reviews, can be applied successfully also to organic ligands. The SIT interaction parameters derived from ligand protonation data for different media, e.g. NaCl and KCl, pass the consistency test when applied to other systems evaluated in the organics review, e.g. solubility data. Hence, the thermodynamic constants selected in this NEA TDB organics review can be used with some confidence in real world applications, provided that SIT is used in the speciation calculations.
The synthesis of the new complexing agents tris‐(2‐pyridylmethyl)‐amine and N,N,N′,N′‐tetrakis‐(2‐pyridylmethyl)‐ethylenediamine is described, and their protonation constants are given.
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