Complex ions stability constants

Compartmental pyrazole ligands (14) can be used to form bimetallic complexes (15).157 This ligand enforces a separation distance between the metal ions. Stability constants were calculated showing a stability for both mononuclear and dinuclear compounds that was less than Cu11 but greater than Nin. 

Table XIX contains stability constants for complexes of Ca2+ and of several other M2+ ions with a selection of phosphonate and nucleotide ligands (681,687-695). There is considerably more published information, especially on ATP (and, to a lesser extent, ADP and AMP) complexes at various pHs, ionic strengths, and temperatures (229,696,697), and on phosphonates (688) and bisphosphonates (688,698). The metal-ion binding properties of cytidine have been considered in detail in relation to stability constant determinations for its Ca2+ complex and complexes of seven other M2+ cations (232), and for ternary M21 -cytidine-amino acid and -oxalate complexes (699). Stability constant data for Ca2+ complexes of the nucleosides cytidine and uridine, the nucleoside bases adenine, cytosine, uracil, and thymine, and the 5 -monophosphates of adenosine, cytidine, thymidine, and uridine, have been listed along with values for analogous complexes of a wide range of other metal ions (700). Unfortunately comparisons are sometimes precluded by significant differences in experimental conditions.

In a potentiometric study in propylene carbonate, using Pb11 or Tl1 as auxiliary ions, stability constants have been determined for a variety of crown ethers. Some results464 are shown in Table 8. They show that the wrap-around ligand dibenzo-30-crown-10 is relatively quite effective, while the 2 1 complexes, presumably of the sandwich type, are favoured for larger lanthanides and smaller crowns. 

The calix[4]azacrowns 41 and 42 (fig. 38), capped with aminopolyamide bridges bind Lnm ions and form both 1 1 and 1 2 (Ln L) complexes with stability constants in the ranges log/3i v 5-6 and log (h 10-11 in acetonitrile for 42, while the stability of the complexes with 41 is about two orders of magnitude smaller. Hydration numbers around 1 were found for the Eum and Tbm complexes, but the ability of the ligands to sensitize Lnm luminescence is veiy weak, except in the Tbm complex with 42. No Er111 luminescence could be evidenced, but some Ndm emission was recorded, which is 12 times larger with ligand 42 than with receptor 41 (Oueslati et al., 2006). 

In microbial iron assimilation, one mechanism for the release of iron from siderophores is the enzymatic reduction to the Fe state. Siderophore stability constants are much lower for Fe +, which has a lower charge-to-radius ratio. Moreover, ligand exchange reactions for the high-spin Fe ion are much faster than for the Fe ion. Stability constants of ferrous siderophores are experimentally difficult to obtain. Limiting pH-independent redox potentials can be utilized, however, to describe the electrochemical and chemical equilibria between fidly coordinated Fe + and Fe +-siderophore complexes and the uncomplexed Fe(H20)6 + and Fe(H20)e +, respectively, in a simple model as described in equation (5) ... 

Linear charge separation on a polyion backbone Microscopic stability constant of MA,-type complex Apparent stability constant of MA-type complex Total concentration of A + ions (mol dm" )... 

As addition of. the-Smrnonia-solutioi Jcqntinued,.-the. free NHsTmoleciJeeTdrsplace the water mole- cules from the pale-blue CulHpO) complex ion tO. form the royal-blue. CulNHa) complex, which is more stable than the water-complex (larger stability constant). i, i- V.,... 

The concentration of (EDTA) ", and thus the ability to complex metal ions, will depend upon the pH. A decrease in pH results in an increase in the deprotonation of EDTA and hence an increase in the concentration of the ED I A ion. The effect of this is that only metal ions with a very high affinity for EDTA will be able to form stable complexes. The stability constants for the EDTA and [diethylenetriaminepentaacetic acid] - (DTPA ) complexes with some important metal ions that are of particular interest for chelation therapy are listed in Table 7.3. It is important to note that the stability of the EDTA and DTPA complexes with toxic metals, such as lead, mercury, cadmium, or plutonium are quite similar to those with essential metals such as zinc, cobalt or copper however, the Ca complex is many orders of magnitude lower. This has important implications for chelation therapy. First, the mobilization and excretion of zinc and other essential metals are likely to be increased, along with that of the toxic metal during EDTA treatment and secondly, the chelation of the ionic calcium in the blood, that can cause tetany and even death, can be avoided by administering the chelator as the calcium salt. 

Ion transport by macrobicyclic ligands (cryptands) has been investigated using a bulk liquid membrane149. It was found that the relative transport rates are not proportional to complex stability. E.g. cryptand (Fig. 67 m = n = 1) which forms a very stable K+ complex, is an inefficient K+ carrier, because of the slow exchange rate of the complex167. On the other hand, cryptand (Fig. 68 3) which forms a more lipophilic and less stable complex, is quite an efficient K+ carrier. It appears that the cryptands display efficient carrier properties for those cations which form complexes with stability constants of about 10s in methanol. 

It is observed in the experiment that the iron nail immediately creates a copper deposit in a blue colored copper sulfate solution (see E8.1), whereby this does not happen in the violet colored ammine complex solution. A trace of copper deposit can only be observed after it has been dipped into the complex solution for a while (see E9.6). It is possible to verify this hypothesis with the help of a second reaction, the metal hydroxide precipitation (see E9.6) a greenish blue deposit is commonly observed in the blue solution of hexaaquacopper ions, but not in the solution of tetraamminecopper ions. Apparently, copper ions and water molecules are not very tightly bonded in aqua complexes, but copper ions and ammonia molecules in ammine complexes are there is a weak stability of aquacopper ions, but a great stability of tetraamminecopper complexes. The stability constants can be taken and interpreted if one wants a quantitative explanation of these phenomena. 

The components of an ion-association aqueous model are (1) The set of aqueous species (free ions and complexes), (2) stability constants for all complexes, and (3) individual-ion activity coefficients for each aqueous species. The Debye-Huckel theory or one of its extensions is used to estimate individual-ion activity coefficients. For most general-purpose ion-association models, the set of aqueous complexes and their stability constants are selected from diverse sources, including studies of specific aqueous reactions, other literature sources, or from published tabulations (for example, Smith and Martell, (13)). In most models, stability constants have been chosen independently from the individual-ion, activity-coefficient expressions and without consideration of other aqueous species in the model. Generally, no attempt has been made to insure that the choices of aqueous species, stability constants, and individual-ion activity coefficients are consistent with experimental data for mineral solubilities or mean-activity coefficients. 

The set of complexes the individual-ion, activity-coefficient parameters and the stability constants are listed in Table II, for each of the three models (1) The WATEQ model, (2) the amended WATEQ model, and (3) the fit model. The WATEQ and amended WATEQ models had the same set of complexes and stability constants. The two models differed only in the individual-ion activity co( fficients of the free ions. The fit model contained different complexes or stability constants or both compared to the WATEQ and amended WATEQ models (except for the OH complexes that were obtained from the WATEQ inodel). The activity coefficients for the free ions were the same in the fit and amended WATEQ models, but the activity coefficients of the complexes differed. 

In the case of palladium, the measured equilibrium potential as a function of the chloride ion concentration (hydrochloric acid) at constant ionic strength (perchloric acid) together with the calculated concentrations of the various complexes using stability constant data leads to the standard poten-... 

At the surface, one should not expect magnesium and calcium to be able to replace zinc in its EDTA complex since the absolute stability constant of zinc is several orders of magnitude higher than the absolute stability constant of the Group II metals. This can be explained by the presence of ammonium ions and the relevant ion stability constants with Eriochrome Black. 

The equilibrium constants for the displacement by iodide ions of the two solvent molecules coordinated along the z axis of the parent complex (the stability constants of the iodo mixed complexes, measured in various solvents) are presented in Table 4.4, together with the Gutmann donicities and the relative permittivities of the solvents. 

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