Stability constants selectivity

The above account of selectivity of inorganic plus organic chemistry in synthesis is given rather extensively to stress three points. All the four (Mg, Fe, Co and Ni) porphyrin products came from one source, the synthesis of uroporphyrin. The basis of selection is very different from that in primitive centres which use thermodynamic stability constant selectivity based on different donor atoms for different metal ions. Here, all ion complexes have the same donor atoms, nitrogen, the most constrained being the coordination of Mg2+ by five nitrogens exactly as is seen for Fe in haemoglobin. Hence, there also has to be a new control feedback to ensure that the appropriate quantities of each metal cofactor is produced in a balanced way, that is synthesis from uroporphyrin has to be divided based upon... 

These were calculated from the analytical solution compositions given in Table III using the computer program MINEQL (Westall, Zachary and Morel (17)) with stability constants selected from Kartell and Smith (18). MINEQL corrects for all hydrolysis and complexation and for computed ionic strength. The resulting values of KgQ are listed in Table III. The solubility data reported by Cameron (15) were rejected because our calculations showed his solutions to be much oversaturated with respect to HgO. 

The interpretation of the titration data given by the authors starts from the premise that Zr(OH)2 is the main hydrolysis species. Moreover, as in [99VEY], the implicit assumption is made that all Zr is bound to carbonate complexes, i.e. a very high formation constant is assumed a priori. Therefore, the stability constant ( "4 = 8.0 x 10 °) derived by [80MAL/CHU] for the formation reaction Zr(C03)j" + CO " = Zr(C03)4, had to be rejected. We re-interpreted the potentiometric data on the base of full equilibrium calculations and using the Zr hydroxo, chloride and sulphate stability constants selected in this review (see below). 

A number of studies of the hydrolytic reactions of cerium(III) have reported stabihty constants for Ce(OH)3(aq) (Fatin-Rouge and Biinzh, 1999 Bentouhami et al., 2004 Teksoz, Acar and Onak, 2009). None of the constants given are consistent with the stability constant selected in this review for CeOH and, as such, are not retained. This is also the case for the constant derived by Bilal and Koss (1981) for Ce(OH)2 and, as such, this datum is also not retained (this will also be the case for other data reported by Bilal and Koss). 

There are very few reported values of the higher monomeric stability constants of gadolinium. The values that have been reported are listed in Table 8.37. There are only four reported values for the stability of Gd(OH)2 and three values for the stability of Gd(OH)g(aq). None of the data appear consistent with the stability constant selected for GdOH " in this review. In all cases, the stability constant would lead to a stepwise stability where either log K2 or log K is greater than or equal to log K. This behaviour is considered unlikely for the lanthanide trivalent metal ions, and consequently, none of the values are retained by this... 

There is only a single literature source that lists data for the stability of the higher monomeric stability constants of terbium (Fatin-Rouge and Biinzli, 1999), where values were proposed for the stability of Tb(OH)3(aq) in two media at 0.1 mol 1 . Again, these data are not consistent with the stability constant selected for TbOH " in this review. In both cases, the stability constant given for log 3 indicates that it is greater than 3-log As indicated previously, this behaviour is considered unlikely, and consequently, neither of the values is retained by this review (the values have not been listed). 

The stability constant determined is fractionally more negative than that given by Lemire et al. (2013) (log j° = —2.15 0.07) but is completely (including the uncertainty interval) within the uncertainty range of that value. It is also in very good agreement with the stability constant selected by Baes and Mesmer (1976) (log ° = —2.19 0.02). From the temperature dependence of the stability constants, the enthalpy of reaction is... 

It would appear apparent from examination of Figure 11.69 that the zero ionic strength stability constant selected in this study is consistent with the data obtained in perchlorate media at fixed ionic strength. The datum from Shankar and De Souza (1963) was obtained at 29 "C it is believed that the difference between the measured constant and that at 25 C is within the uncertainty assigned to the constant. 

Table 3. Thermodynamic and Stability Constant Data for Selected Aqueous Cadmium Complexes ...

Chaput, Jeminet and Juillard measured the association constants of several simple polyethylene glycols with Na", K", Cs", and Tl". Phase transfer catalytic processes and most biological processes are more likely to involve the first two cations rather than the latter two, so we will confine the discussion to these. Stability constants for the dimethyl ethers of tetra-, penta-, hexa-, and heptaethylene glycols were determined poten-tiometrically in anhydrous methanol solution and are shown in Table 7.1. In the third column of the table, the ratio of binding constants (Ks/K s) is calculated. Note that Simon and his coworkers have referred to this ratio as the selectivity constant. ... 

By considering the stability constant and the lipophilicity of host molecules, Fyles et al. synthesized a series of carboxylic ionophores having a crown ether moiety and energetically developed the active transport of alkali metal cations 27-32). Ionophores 19-21 possess appropriate stability constants for K+ and show effective K+-selective transports (Fig. 5). Although all of the corresponding [15]crown-5 derivatives (22-24) selectively transport Na+, their transport rates are rather slow compared with... 

EDTA and NTA are essentially the only two chelants formulated into BW deposit control programs. The rationale for selection of one product over another is often a point of contention. Looking first at the relative chelation stability constants (chelate log values, Ks) ... 

Complexes with the Fluoride Ion. For the compilation of stability constants (Table IV) of complexes with F , we have used, when needed, thermodynamic parameters (K, AH) pertaining to the dissociation of hydrofluoric acid as given by Smith and Martell (77) or extrapolated from their selection. 

To select the metal to be incorporated into the substrate porphyrin unit, the following basic properties of metalloporphyrins should be considered. The stability constant of MgPor is too small to achieve the usual oligomeric reactions and purification by silica gel chromatography. The starting material (Ru3(CO)i2) for Ru (CO)Por is expensive and the yield of the corresponding metalation reaction is low. Furthermore, the removal of rutheniirm is difficult, and it is likewise difficult to remove the template from the obtained ruthenium CPOs. Therefore, ZnPor is frequently used as a substrate in this template reaction, because of the low prices of zinc sources (zinc acetate and/or zinc chloride), the high yield in the metalation reaction, the sufficient chemical stability of the ZnPor under con-... 

The coupling reaction of 1 (M=Zn) affords CPO 3 (M=Zn) in 55% yield in the presence of template 2 however, the absence of 2 decreases the yield to 34% [22]. With the increase of yield of 3, template 2 induces the selectivity of the reaction the yield of the by-product (cychc dimer 4 (M=Zn)) was changed from 23% (with no template) to 6% (in the presence of template). A similar CPO formation reaction was reported for the corresponding ruthenium porphyrins (3, M=Ru(CO)), in which the stability constant of the Ru-N coordination bond is 10 larger than that of the Zn-N coordination bond [23]. Although the transition state of the CPO produced by the ruthenium-based substrate is expected to be more stable than that produced by ZnPor, the yield of 3 (M=Ru(CO)) is only... 

The chelate HgY2 must be considerably more stable than MY( l 4)+ if necessary one can mask M"+ to a certain desirable extent by means of a selective com-plexing agent A, so that KM becomes an apparent stability constant,... 

Smith, R. M. Martell, A. E. Motekaitis, R. J, NIST Critically Selected Stability Constants of Metal Complexes Database, Version 5, NIST Standards Reference Database 46, U. S. Dept. Commerce, Gaithersburg, MD, 1998. 

Especially sensitive and selective potassium and some other ion-selective electrodes employ special complexing agents in their membranes, termed ionophores (discussed in detail on page 445). These substances, which often have cyclic structures, bind alkali metal ions and some other cations in complexes with widely varying stability constants. The membrane of an ion-selective electrode contains the salt of the determined cation with a hydrophobic anion (usually tetraphenylborate) and excess ionophore, so that the cation is mostly bound in the complex in the membrane. It can readily be demonstrated that the membrane potential obeys Eq. (6.3.3). In the presence of interferents, the selectivity coefficient is given approximately by the ratio of the stability constants of the complexes of the two ions with the ionophore. For the determination of potassium ions in the presence of interfering sodium ions, where the ionophore is the cyclic depsipeptide, valinomycin, the selectivity coefficient is Na+ 10"4, so that this electrode can be used to determine potassium ions in the presence of a 104-fold excess of sodium ions. 

Fluoride ion selective spectrometry was used to determine the stability constants for zinc fluoride complexes in water at 25 °C, giving values j3i[ZnF(aq)]+ = 3.5 0.1 and /l2[ZnF2(-aq)] = 3.8 0.5.643 These results demonstrate that the complexation of fluoride is very weak and in aqueous chemistry no species beyond ZnF+ is of much importance. Organotitanium fluorides have been used as matrices for trapping molecular ZnF2 and MeZnF. 4... 

The ligand 6,13-dimethyl-l,4,8,ll-tetra-azacyclotetradecane-6,13-diamine coordinates as a hexadentate ligand to zinc in neutral aqueous solution. Potentiometric titrations were used to determine the stability constant for formation. The pXa values were determined for five of the six possible protonation steps of the hexamine (2.9, 5.5, 6.3, 9.9 and 11.0).697 Studies of the syn and anti isomers of 6,13-dimethyl-1,4,8, ll-tetraazacyclotetradecane-6,13-diamine reveal that they offer different shapes for metal binding, which is reflected in the stability constants for 1 1 zinc ligand ratio complexes. The selectivity of binding to the zinc ion compared to the cadmium(II) ion by both isomers is significant.698... 

There are many types of compounds that form colored complexes with metal ions. The color reaction must be sufficiently selective and the value of the stability constant of the complex formed should be such as to make the reaction reversible in order to make the device a sensor rather than a singleshot probe1 3 18 43 50. 

There are two main factors that influence the selectivity of a sensor limits in discrimination of an interfering ion and upper limits in stability constant of an analyte-ionophore complex. While an ideal ionophore does not form complexes with interfering ions, too strong complexation with the primary ion leads to a massive extraction of analyte into membrane phase coupled with a coextraction of sample counter-ions, known as Donnan exclusion failure. In such cases, at high activities and lipophilicities of sample electrolytes, fli(org) increases and a breakdown of membrane permselectivity prevents the Nemst equation to hold. 

M.M. Shultz, O.K. Stefanova, S.S. Mokrov, and K.N. Mikhelson, Potentiometric estimation of the stability constants of ion-ionophore complexes in ion-selective membranes by the sandwich membrane method theory, advantages, and limitations. Anal. Chem. 74, 510-517 (2002). 

Calorimetric studies indicate that the enthalpies of complexation tend to show related trends to the observed stability constants and display selectivity peaks, although there is not necessarily a coincidence between the two sets of peaks. Complexation is characterized by the entropy becoming progressively less positive (less favourable) as the cation size decreases. This is illustrated in Figure 6.5 for the complexation of 2.2.1 with the alkali metals. 

The dissociation rates for a number of alkali metal cryptates have been obtained in methanol and the values combined with measured stability constants to yield the corresponding formation rates. The latter increase monotonically with increasing cation size (with cryptand selectivity for these ions being reflected entirely in the dissociation rates - see later) (Cox, Schneider Stroka, 1978). 

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. 

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