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• W1992423909 abstract "The kinetics of the substitution reactions of the metal ions in the lanthanide(III)-ethylenediaminetetraacetate (Lnedta− = LnY) complexes have been investigated in detail by isotopic-exchange [1-4] the main features of the mechanisms of the reactions are known. The exchange can take place by the proton catalyzed dissociation of complexes as well as by the direct attack of the exchanging metal on the complex. In the interval <pH <6 the dissociative path perdominates and it is assumed that the relatively slow dissociation of complexes is followed by a fast reaction of the free ligand with the exchanging metal. The intimate mechanisms of these processes are not known and the possible role of the water-exchange rate of the metal ions has hardly been investigated. Recently studying the kinetics of the exchange reactions between the Ceedta and Ni2+ or Co2+ ions we have shown [7] that the reactions of the free ligand formed by dissociation, depend on the water-exchange rate of both outcoming and incoming metal ions. As a result the dissociation of the complex must be regarded as reversible reaction as it is described in the following scheme: where the degree of protonation of the ligand, i, depends on the pH of the solution. In the presence of Ce3+ and M2+ ion excess pseudo-first-order rate constants kp values on the concentration of the H+, Ce3+ and M2+ ions, the formation rate constants kMHiY of the Medta2− complex can be calculated. [7]. With the application of this principle the formation rate constants of some Lnedta complexes have been determined, since in spite of the interest of these complexes their rate of formation has not been investigated [6]. For the determination of the formation rate constants kLnHiY the kinetics of the following exchange reaction was investigated: Ceedta− + Ln3+⇌Ce3+ + Lnedta− where Ln = Nd, Gd, Er and Y. The formation rate constants can be determined only when the rate of formation of the Lnedta complexes can be slowed down with the increase of the concentration of the Ce3+ ions. This condition fulfills if k−H2OCe[Ce3+] > k−H2OLn[Ln3+]/10 (k−H2OCe and k−H2OLn are the characteristic water-exchange rate constants of the Ce3+ en the Ln3+ ions). The exchange reactions (4) were followed by spectrophotometry at 280 nm (25 °C, I = 1 M KCl, cell length 4 cm). The concentration of the Ceedta was 4 × 10−4M, while the pH as well as the concentration of the Ln3+ ions was always high enough ([Ln3+]>[Ce3+]) to make the exchange practically complete. With the application of these conditions the exchange reaction (40 can be treated as a first order one and its rate can be expressed as follows: − d[CeY]tdt = kp[CeY]t Taking into consideration all the possible reaction paths of the exchange the pseudo-first-order rate constant can be expressed with the following equation: where kLnCeY and kLnCeHY are the rate constants of the reactions taking place by the direct attack of the Ln3+ ions on the Ceedta− and CeHedta complexes. KCeY is the stability constant, KHCeY is the protonation constant of Ceedta−. βi is the i-th protonation constant of the edta4−ligand. The formation rate constants have been calculated using eqn. (6) to computer fit the kp values obtained at various H+, Ce3+ concentrations. The results of the calculations indicated and appropriate fitting with the use of i = 1 and 2. The contribution t001. The formation rate constants of the Lnedta complexes and the kLnCeY values.(kM−1 s−1)NdGdErY108 × kLnHY2.73.01.80.9106 × kLnH2Y1.11.10.540.18kLnCeY0.350.280.220.14 of the reaction taking place by the direct attack of the Ln3+ ions on the protonated complex CeHedta has some importance only in the reaction with Y3+ (kYCeHY = 200 M−1 s−1). The rate constants obtained are listed in Table I. t001. The formation rate constants of the Lnedta complexes and the kLnCeY values. If the formation of complexes takes place by the Eigen-mechanisms, the second-order rate constants can be expressed as the product of the water-exchange rate constant of the metal ion and the outer sphere association constant Kos of the ions: kLnHY = k−H2OLn × Kos. The water-exchange rate constants of the Ln3+ ·aq ions are quite uncertain but their order of magnitude is about 108 [6]. A more accurate value is known for the Gd3+ ·aq, k−H2OGd = 10· × 108 s−1 [8]. Taking into account the value kGdHY = 3 × 108M−1 s−1 (Table I) the association constant Kos = 0.28 M−1 For 3+ and 3− ions the predicted Kos value is about 100, that is much higher [9]. This suggests that the formation rate constants obtained are too low to assume the rate determining role of the water-exchange from the first coordination sphere. It seems more probable that the rate controlling step in the reaction between the Ln3+ ion and the monoprotonated Hedta3− ligand is the ring closure because one of the iminodiacetate groups of the ligand is deprotonated. The kLnH2Y values are about 2 orders of magnitude lower than the kLnHY values which can be interpreted assuming a slower, rate controlling proton transfer step (or deprotonation) making possible the coordination of a deprotonated iminodiacetate group." @default.
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• W1992423909 date "1984-02-01" @default.
• W1992423909 modified "2023-10-17" @default.
• W1992423909 title "Kinetics of the exchange reactions and the formation rate of the lanthanide(III)-ethylenediamine-tetraacetate complexes" @default.
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• W1992423909 doi "https://doi.org/10.1016/s0020-1693(00)94527-4" @default.
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