Complex Stability Constant Effect

Figure 6.10. Complex formation of Fe(III) and of Cu(II) by various ligands The Lability cannot be predicted alone from complex stability constants but competitive effects of (with metal ions) and of OH (with ligands) need to be considered. Multidentate complex formers form more stable complexes, especially at high dilutions, than monodentate ligands (e.g., F, NH3). To solutions of TOTFe(III) = 10 M and TOTCu(II) = 10 M, respectively, complex formers (at the concentrations indicated in the figures) were added (points are calculated). (Cit = citrate, gly = gly

If desolvation of the anionic nucleophile is to be the prominent factor in the molecular catalysis one would expect the most heavily solvated guest to display the largest rate enhancement factors at saturation in a given type of reaction. Heavy solvation of the anion may, however, impair host-guest complex formation so that the complete kinetic analysis with evaluation of every rate- or complex stability constant might not be possible. This turned out to be the case when the rate effects of different anionic nucleophiles in aromatic nucleophilic substitutions were studied The catalysis by the polyammonium host molecule 25 in most of these reactions could only be characterized by a Fig. of merit (FM = K ke ) but not by individual rate constants. 

Discussion of the effect of ligand structure on protein-carbohydrate affinity requires an evaluation of complex stability constants. A munber of biophysical techniques are appropriate for the study of protein-carbohydrate interaction many of the more enlightening strategies are the topics of separate chapters elsewhere in this volume. We describe below three techniques used extensively in glycobiology— inhibition of hemagglutination, enzyme-linked lectin assay (ELLA), and isothermal titration microcalorimetry—and we consider the types of information provided by each technique in order to facilitate appropriate interpretation of the data. 

NMR signals of the amino acid ligand that are induced by the ring current of the diamine ligand" ". From the temperature dependence of the stability constants of a number of ternary palladium complexes involving dipeptides and aromatic amines, the arene - arene interaction enthalpies and entropies have been determined" ". It turned out that the interaction is generally enthalpy-driven and counteracted by entropy. Yamauchi et al. hold a charge transfer interaction responsible for this effect. 

In view of the magnitude of crystal-field effects it is not surprising that the spectra of actinide ions are sensitive to the latter s environment and, in contrast to the lanthanides, may change drastically from one compound to another. Unfortunately, because of the complexity of the spectra and the low symmetry of many of the complexes, spectra are not easily used as a means of deducing stereochemistry except when used as fingerprints for comparison with spectra of previously characterized compounds. However, the dependence on ligand concentration of the positions and intensities, especially of the charge-transfer bands, can profitably be used to estimate stability constants. 

In equation (q) only the fully ionised form of EDTA, i.e. the ion Y4 , has been taken into account, but at low pH values the species HY3, H2Y2, H3 Y and even undissociated H4Y may well be present in other words, only a part of the EDTA uncombined with metal may be present as Y4. Further, in equation (q) the metal ion M"+ is assumed to be uncomplexed, i.e. in aqueous solution it is simply present as the hydrated ion. If, however, the solution also contains substances other than EDTA which can complex with the metal ion, then the whole of this ion uncombined with EDTA may no longer be present as the simple hydrated ion. Thus, in practice, the stability of metal-EDTA complexes may be altered (a) by variation in pH and (b) by the presence of other complexing agents. The stability constant of the EDTA complex will then be different from the value recorded for a specified pH in pure aqueous solution the value recorded for the new conditions is termed the apparent or conditional stability constant. It is clearly necessary to examine the effect of these two factors in some detail. 

The extent of hydrolysis of (MY)(n 4)+ depends upon the characteristics of the metal ion, and is largely controlled by the solubility product of the metallic hydroxide and, of course, the stability constant of the complex. Thus iron(III) is precipitated as hydroxide (Ksal = 1 x 10 36) in basic solution, but nickel(II), for which the relevant solubility product is 6.5 x 10 l8, remains complexed. Clearly the use of excess EDTA will tend to reduce the effect of hydrolysis in basic solutions. It follows that for each metal ion there exists an optimum pH which will give rise to a maximum value for the apparent stability constant. 

Salts of diazonium ions with certain arenesulfonate ions also have a relatively high stability in the solid state. They are also used for inhibiting the decomposition of diazonium ions in solution. The most recent experimental data (Roller and Zollinger, 1970 Kampar et al., 1977) point to the formation of molecular complexes of the diazonium ions with the arenesulfonates rather than to diazosulfonates (ArN2 —0S02Ar ) as previously thought. For a diazonium ion in acetic acid/water (4 1) solutions of naphthalene derivatives, the complex equilibrium constants are found to increase in the order naphthalene < 1-methylnaphthalene < naphthalene-1-sulfonic acid < 1-naphthylmethanesulfonic acid. The sequence reflects the combined effects of the electron donor properties of these compounds and the Coulomb attraction between the diazonium cation and the sulfonate anions (where present). Arenediazonium salt solutions are also stabilized by crown ethers (see Sec. 11.2). 

However, an evaluation of the observed (overall) rate constants as a function of the water concentration (5 to 25 % in acetonitrile) does not yield constant values for ki and k2/k i. This result can be tentatively explained as due to changes in the water structure. Arnett et al. (1977) have found that bulk water has an H-bond acceptor capacity towards pyridinium ions about twice that of monomeric water and twice as strong an H-bond donor property towards pyridines. In the present case this should lead to an increase in the N — H stretching frequency in the o-complex (H-acceptor effect) and possibly to increased stabilization of the incipient triazene compound (H-donor effect). Water reduces the ion pairing of the diazonium salt and therefore increases its reactivity (Penton and Zollinger, 1971 Hashida et al., 1974 Juri and Bartsch, 1980), resulting in an increase in the rate of formation of the o-complex (ik ). 

Complexation of Pu is discussed in terms of the relative stabilities of different oxidation states and the "effective" ionic charge of Pu0 and Pu02+2. An equation is proposed for calculating stability constants of Pu complexes and its correlation with experimental values demonstrated. The competition between inner v outer sphere complexation as affected by the oxidation state of Pu and the pKa of the ligand is reviewed. Two examples of uses of specific complexing agents for Pu indicate a useful direction for future studies. 

One of several studies of this system shows remarkable agreement with the present results. Fardy and Pearson (3) investigated this system by cation exchange at 2 M acidity and ionic strength and reported uncorrected stability constants 8i = 278 (+ 8) and B2 = 6.8 (+0.2) x 103 (K2 24). Assuming the difference in the medium (2 M HClOi, vs. 1 M NaClO, 1.0 M HClOi ) has a minimal effect on the activities of the various complexes,... 

Transition-metal complexes span an enormous range of stabilities. One of the principal aims of this chapter is to attempt to understand some of the factors which control these, and to determine the importance of ligand-field effects. Very extensive compilations of stability constants are available. 

The Chelate Effect and Polydentate Ligands 147 Table 8-1. Stability constants for some nickel(ii) complexes of ammonia and 1,2-diaminoethane. 

Table 6. Free calcium concentrations in equilibrium with common complexing agents. A low free calcium concentration implies effective complexation, whether the complex formed is soluble or insoluble. The data were derived from either stability constants (soluble complexes) or solubility products (insoluble complexes).

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