Stability of metal chelates

In order to understand the interactions of proteins with metals, it is necessary to consider the effect of pH on the stability of metal chelates.78,78 The following, simplified treatment illustrates the equilibria that can occur between a metal ion, a chelating agent (or other ligand), and hydrogen ion. Although it is almost impossible to apply the equations quantitatively to metalloenzymes, they do illustrate the interactions of the different variables. 

Comparison of Stabilities of Metal Chelates of Aminopolyacetate and Aminopolymethylenephosphonate Ligands ... 

Good stability of metal chelate and, connected to this, the selectivity toward different ions, which arc both strongly affected by the nature of the Mannich ba.se, are reported Table 39 lists the main metal ions giving complexes with different Mannich bases. 

Recently a large amount of experimental data on stability constants (17) of metal chelates and associated heats and entropies of formation has revealed that many factors enhance the stabilities of metal chelates in solution. This paper examines each of these factors and discusses their significance. 

Recently new data have become available on the stabilities of metal chelates formed from NTA and EDTA analogs, represented by XVIII, through the publication of equilibrium data on the chelates of two higher members of the series, triethylenetetraminehexaacetic acid (TTHA) and tetraethylenepentamineheptaacetic acid (TPHA) (5, 4)- The stabilities of some representative metal chelates of the series of ligands represented by Formula XVIII are listed in Table VI. 

Potentiometric H2O t = lS 2 c = 0.04 Holmes F and Crimmin WRC, The stabilities of metal chelate compounds formed by some heterocyclic acids. I. Studies in aqueous solution,/. Chem. Soc., 1175-1180 (1955). 

Gustafson RL and Martell AE, Stabilities of metal chelates of pyridoxamine. Arch. Biochem. Biophys., 68, 485-498 (1957). CA 51 85010. Cited in Perrin Bases No. 3332 ref. G62. 

The second very important entropy-induced effect is the great stability of metal chelates (see definition of chelate in Section 1.3). Both ammonia and ethylenediamine (en) coordinate with metals through amine nitrogens in terms of the heat evolved in the complexation reaction two molecules of NH3 have been shown to be about equivalent to one en molecule. However, en eomplexes are considerably more stable than their NH3 eounterparts for example, [Ni(NH3)6], KiK2 = 6x 10 K3Ki = 5 x 10 KsK = 3. [Ni(en)3f, Ki = 5x 10 K2 = 2.2 X 10 K2 = 3.6x 10 ). It has been experimentally demonstrated that the unusual stability of the en compounds is due to a more favorable entropy associated with their formation. 

Thiophen Derivatives of Analytical Interest.—2-Thenoyltrifluoroacetone has maintained its position as a chelating agent in analytical chemistry. Papers describing its use in the extraction or determination of thorium, copper, europium, thallium, niobium, and molybdenum have appeared. The effect of copper(n) on the formation of monothenoyltri-fluoroacetonatoiron(iii) has been studied. The stability constants of some bivalent metal chelates of di-(2-thenoyl)methane have been determined. 3-Thianaphthenoyltrifluoroacetone has been proposed as a reagent for the spectrophotometric determination of iron(iii) and cerium(iv). The stabilities of metal chelates formed from derivatives of thiophen-2-aldehyde and of rare-earth carboxylates of thiophen-2-carboxylate have been studied. 

The factors which influence the stability of metal ion complexes have been discussed in Section 2.23, but it is appropriate to emphasise here the significance of the chelate effect and to list the features of the ligand which affect chelate formation ... 

The type of catalyst influences the rate and reaction mechanism. Reactions catalyzed with both monovalent and divalent metal hydroxides, KOH, NaOH, LiOH and Ba(OH)2, Ca(OH)2, and Mg(OH)2, showed that both valence and ionic radius of hydrated cations affect the formation rate and final concentrations of various reaction intermediates and products.61 For the same valence, a linear relationship was observed between the formaldehyde disappearance rate and ionic radius of hydrated cations where larger cation radii gave rise to higher rate constants. In addition, irrespective of the ionic radii, divalent cations lead to faster formaldehyde disappearance rates titan monovalent cations. For the proposed mechanism where an intermediate chelate participates in the reaction (Fig. 7.30), an increase in positive charge density in smaller cations was suggested to improve the stability of the chelate complex and, therefore, decrease the rate of the reaction. The radii and valence also affect the formation and disappearance of various hydrox-ymethylated phenolic compounds which dictate the composition of final products. 

Chitosan (> 75% deacelylation, 800-2000 cps) was mixed wilh stock so-lulions of Cu(II), Fe(ll), Cd(ll) and Zn(II), prepared in 0.1 M HNO3, and of Ca(ll) and Mn(II), in 0.1 MHCl. It was found that, in the chelation of most metal ions by chitosan, 1 1 binding of chitosan is more dominant than 2 1 cooperative binding, but vice versa for Zn(II) and Cd(II). The chelation of Cu(II) by chitosan showed much higher reactivity when compared to other divalent metal ions. Cu(II), Fe(II), Cd(II) andZn(II) showed strong reactivity and stability of their chelates. In contrast, the interactions between Ca(II) or Mn(II) and chitosan were almost negligible. These data confirm brilliantly previous data by Muzzarelli et al. [116]. 

The preparation of neutral chelates for the separation of etals by gas chromatography has been studied for many years [436,667,669-672]. The principal limit to the success of this approach is the paucity of suitable reagents which can confer the necessary volatility, thermal stability and chemical inertness on the metal ions. The class of metal chelating reagents that have... 

Instead of metal chelation, an intramolecular hydrogen bonding between the oxygen atom of phenolate and a hydrogen atom of a carboxylic acid in the 8-position leads to stabilization of the colored form, such as compound 12.20,21 This spiropyran exhibits reversed photochromism, which means that thermally stable species change from the spiro form to the colored form, and thus the colorless form produced by photoirradiation soon converts to thermally stable colored form. 

Colored forms of 5, 8 -disulfonate derivatives of 33 chelate with divalent metal ions, e.g., Ca2+, Cu2+, and Pb2+, causing blueshift.85 The order of blueshift and thermal stability of the chelated photomerocyanine is as follows Ca2+ < Cu2+ < Pb2+. 5 -Methoxy derivative of 33 also gives Ni2+ complexes. This chelation significantly stabilizes photomerocyanine, compared with the nonchelating colored form. In contrast to sulfonate derivatives, chelation of 5 -methoxy derivatives with Ni2+ causes redshift (ca. 40 nm), but their structures are not clear. 

Figure 10.6 Effect of pH on the conditional stability constants at 25 °C of metal chelates of DTPA [20]...

The extractants TBOA, TBMA, and TBSA are very similar, but their structural differences (see formulas in Table 4.16) allow the formation of only one type of metal chelate complex 5-, 6-, and 7-membered rings, respectively. Similar-ily, the reactants DMDOMA and DMDOSA form only 6- and 7-membered chelates. Table 4.16 shows that extraction (i.e., largest Ke value) is favored by 6-membered rings. This is not unexpected as the values in this case reflect the stability constants (3 acc. to Eq. (4.74). 

Also, some distillate fuel stabilizers contain metal chelating agents. The purpose of this is to help prevent the metal-catalyzed oxidation of fuel components. However, if a distillate fuel containing a chelating agent is stored in a tank or transferred through lines which are rusted, the stabilizer may act to chelate iron. As a result of this, the fuel containing the stabilizer appears dark. 

Eichhom and his co-workers have thoroughly studied the kinetics of the formation and hydrolysis of polydentate Schiff bases in the presence of various cations (9, 10, 25). The reactions are complicated by a factor not found in the absence of metal ions, i.e, the formation of metal chelate complexes stabilizes the Schiff bases thermodynamically but this factor is determined by, and varies with, the central metal ion involved. In the case of bis(2-thiophenyl)-ethylenediamine, both copper (II) and nickel(II) catalyze the hydrolytic decomposition via complex formation. The nickel (I I) is the more effective catalyst from the viewpoint of the actual rate constants. However, it requires an activation energy cf 12.5 kcal., while the corresponding reaction in the copper(II) case requires only 11.3 kcal. The values for the entropies of activation were found to be —30.0 e.u. for the nickel(II) system and — 34.7 e.u. for the copper(II) system. Studies of the rate of formation of the Schiff bases and their metal complexes (25) showed that prior coordination of one of the reactants slowed down the rate of formation of the Schiff base when the other reactant was added. Although copper (more than nickel) favored the production of the Schiff bases from the viewpoint of the thermodynamics of the overall reaction, the formation reactions were slower with copper than with nickel. The rate of hydrolysis of Schiff bases with or/Zw-aminophenols is so fast that the corresponding metal complexes cannot be isolated from solutions containing water (4). 

Fig. 3.2.1 Relationship between the stability constants of metal chelates and the corresponding yields of metal sulfide particles including CdS, ZnS, and PbS. The symbols bound by thin, thick, and dotted frames show the yields at different conditions of (a) 25°C, 2 min (b) 25°C, 1 h and (c) 60r C. 8 h, respectively. The solid line curves represent the yields at conditions (a) and (c), while the broken line curve is the upper limit of the yields at conditions (b). (From Ref. 6.)...

Some factors that influence the stability of polymer chelates should be mentioned. Hojo et a/.61) have reported the effect of the ligand ratio [ligand]/[metal ion] on the formation of the Cu chelate of poly(vinylalcohol)(PVA). Figure 10 shows the relationship between the formation constant of the Cu complex, the viscosity of an aqueous solution of PVA, and the ligand ratio. The viscosity diminishes very sharply at about [PVA]/[Cu] = 32 this corresponds to an increase in the formation constant. A tightly packed conformation of PVA, caused by intra-polymer chelation with Cu, facilitates more and more chelate formation. 

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