Stability constants correlations

Activation parameters for the path corresponding to the A i[Fe +]term are — 24.2 0.5 kcal mol and IsS — 8.9 1.6caldeg mol The iron(m) assists the oxalate group to leave by co-ordinating to it, possibly in a unidentate-oxalate intermediate rather than in the starting complex itself. Several examples of metal-ion-catalysed aquation of chromium(ra) complexes are included in the general rate constant against stability constant correlation discussed in the section on catalysed aquation of cobalt(m) complexes. 

Rh(NH3)5X] +, where X = Cl, Br, or I, are also included in Table 13. The suggested mechanism involves slow dissociation of an adduct [Rh(NH3)s-XHg] + which is in equilibrium with the starting reactants. Both Hg +- and HgCl+-catalysed aquations of rhodium(ni)-halide complexes can be incorporated in the general kinetic versus stability-constant correlation for metalion catalysis of aquation of halogeno-complexes mentioned in Section 1 of this chapter. Silver(i) is not an efficient catalyst for the aquation of [Rh(NH3)5-Q]2+ 240 Redox catalysis of substitution at rhodium(m) has been reviewed. ... 

Good to excellent Hammett plots were obtained using substituent constants (see Figure 2.6). Surprisingly, literature examples of good Hammett correlations of stability constants are rare The p-values are shown in Table 2.7. 

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. 

If the photoequilibrium concentrations of the cis and trans isomers of the photoswitchable ionophore in the membrane bulk and their complexation stability constants for primary cations are known, the photoinduced change in the concentration of the complex cation in the membrane bulk can be estimated. If the same amount of change is assumed to occur for the concentration of the complex cation at the very surface of the membrane, the photoinduced change in the phase boundary potential may be correlated quantitatively to the amount of the primary cation permeated to or released from the membrane side of the interface under otherwise identical conditions. In such a manner, this type of photoswitchable ionophore may serve as a molecular probe to quantitatively correlate between the photoinduced changes in the phase boundary potential and the number of the primary cations permselectively extracted into the membrane side of the interface. Highly lipophilic derivatives of azobis(benzo-15-crown-5), 1 and 2, as well as reference compound 3 were used for this purpose (see Fig. 9 for the structures) [43]. Compared to azobenzene-modified crown ethers reported earlier [39 2], more distinct structural difference between the cis... 

Cununing GL, Rollett JS, Rosotti FJC, Whewell RJ (1972) Statistical methods for the computation of stability constants, I. Straight-line fitting of points with correlated errors. J Chem Soc Dalton Trans 23 2652-2658... 

Bioavailable trace elements in soil correlate with plant uptake and concentrations in plants. Extractants for bioavailable trace elements include chelating agents, diluted inorganic acid, neutral salt solutions, and water (Table 7.2). The most popular extractant for bioavailable trace elements in arid and semi-arid soils is DTPA-TEA (triethanolamine), which was developed by Lindsay and Norvell (1969, 1978) to extract available Cu, Zn, Fe and Mn from neutral and calcareous soils. Use of this chelating agent, DTPA, is based on the fact that it has the most favorable combination of stability constants for simultaneous complexation of Cu, Fe, Mn and Zn... 

Many correlations have been made between the stability constants for a series of complexes and other properties. For example, basicity of the ligands, ionic radii, dipole moments, and other properties have been correlated with the stability constants of the complexes. However, before comparisons such as these are made, the stability constants should be corrected statistically to take into account the fact that successive complexes do not have the same probability of forming. 

A measure of the Lewis acidity of a metal ion is determined by its affinity for a pair of electrons, and the greater this affinity, the more stable the complexes formed by the metal ion will be. However, removing electrons from a metal to produce an ion is also related to the attraction the metal atom has for electrons. Therefore, it seems reasonable to seek a correlation between the stability constants for complexes of several metals with a given ligand and the total energy necessary for ionization to produce the metal ions. The first-row transition metal ions react in solution with ethylenediamine, en, to form stable complexes. We will consider only the first two steps in complex formation, which can be shown as follows ... 

Correlation between complex stability and in vivo toxicity has always been an important question in contrast agent development. Conditional stability constants were often correlated to the selectivity of a given ligand for Gdm over endogenous metals, such as Zn11 or Cu11 (134), and therefore they are widely used to compare the behavior of different complexes at physiological pH. 

Fig. 7.3. Correlation between the selectivity coefficient and the ratio of stability constants in water for a liquid membrane ISE based on valinomycin dissolved in nitrobenzene. (After Morf [151].)...

This correlation also exists for ligands with more than one binding site and for successive stability constants. In Fig. 3.5 the log and log 2 values for U02 complexation correlate well with the sum for series of... 

Redox potentials of pentacyanoferrates are often determined in association with kinetic and stability constant determinations. They are also available for 4-methyl- and 4-amino-pyri-dine pentacyanoferrates, and for [Fe(CS )5(2,6-dimethylpyrazine)] in acetonitrile-water mixtures.Oxidation potentials of [Fe(CN)5L] complexes correlate with the electron-withdrawing or -releasing properties of the ligands... 

It is often difficult to measure stability constants directly for Fe + complexes of natural and model siderophores, since their very high stabilities mean there is extremely little free Fe + in equilibrium with the complex. It is therefore common to use the link between redox potentials and stability constants in the two oxidation states involved in estimating values for logAiFeinL- The correlation of logAlpeniL with redox potential, established over a range of 10 in can thus... 

At present, the correlation contains one transition metal complex, Cu(Hfacac)2. The results on this complex are very interesting and somewhat unusual for a transition metal system in that enthalpies have been obtained in a poorly solvating solvent with nonionic donors (52), instead of the t5 ical stability constant study on a metal cation in some highly polar solvent. Data from this latter type of investigation have many practical uses, but are impossible to interpret and understand. The transition metal ion complex we have studied can be incorporated into the E and C scheme using the same base parameters that are used to correlate the enthalpies of formation of all the other Lewis acid-base adducts in the scheme. 

Correlation of Potentials to Copper(ll) Complex Stability Constants... 

Tab.4 Correlation of Cu(ll/I) redox potentials and stability constants of copper complexes in aqueous solution at 25 °C, jx = 0.1...

Dale Margerum Ralph Wilkins has mentioned the interesting effect of terpyridine on the subsequent substitution reaction of the nickel complex. I would like to discuss this point—namely the effect of coordination of other ligands on the rate of substitution of the remaining coordinated water. However, before proceeding we should first focus attention on the main point of this paper-which is that a tremendous amount of kinetic data for the rate of formation of all kinds of metal complexes can be correlated with the rate of water substitution of the simple aquo metal ion. This also means that dissociation rate constants of metal complexes can be predicted from the stability constants of the complexes and the rate constant of water exchange. The data from the paper are so convincing that we can proceed to other points of discussion. 

---