Divalent cation complexes stability

Figure 4. Stabilities of some class B divalent cation complexes at I = 0 based on published data. EN values for Hg Cd Pb Sn and Zn are 1.8,1.5, 1.6, 1.7, and 1.5, respectively (29).

Figure 3.7 Stabilities of some class B divalent cation complexes are shown at zero ionic strength, based on published data. Ligand EN values are 1 (2.66), Br (2.96), Cl- (3.16), F- (3.98), OH" (3.1), and SH (2.33). Cation EN values all range from 1.6 to 1.7, except EN = 2.00 for Hg -. Reprinted with permission from Techniques of estimating thermodynamic properties for some aqueous complexes of geochemical interest, D. Langmuir. In Chemical modeling in aqueous systems, ed. E. A. Jenne, Am. Chem. Soc. Symp. Sen 93. Copyright 1979 American Chemical Society.

The pea leaf glutamine synthetase appears less stable than the pea seed enzyme (O Neal and Joy, 1973b). These workers found that Mg " " and Mn + increased stability of crude and partly purified enzyme, Mn " " being more effective than Mg + especially at low pH. Stability was further improved by addition of sucrose (0.5-1.5 M), fructose (1.3-2.4 M), or ethylene glycol (10-40%). Highly purified enzyme was more stable without Mg " " if molar sucrose or 20-30% ethylene glycol was present. Mercaptoethanol enhanced stability at high (35°-50"C) but not low (0 -3 C) temperature in the presence of Mg + but decreased activity in the presence of Mn " ", except in the presence of ATP. ATP at concentrations which can complex all the divalent cations decreases stability but if Mg or Mn + are present in excess ATP increases stability. 

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. 

The effect of the nature of the divalent cation is very pronounced as illustrated in Figure 2 on sample A30. Pectins were found to be much more sensitive to copper than to calcium. A scale of affinity towards divalent cations can be easily obtained this way [18]. This result corroborates what has been measured by pH titration upon addition of increasing amount of cations [28,29], where the order of decreasing selectivity was Pb = Cu Zn > Cd = Ni > Ca. This scale does not follow the size of the radius of the cations but is in agreement with the sequence of complex stability of Irving-Williams [30]. 

Some of these divalent cations form part of the Irving-Williams series Mn, Fe, Co, Ni, Cu and Zn. Irving Williams (1953) examined the stability constants of complexes of a number of divalent ions and found that the order... 

Setting times and hydrolytic stability of these cements are given in Table 8.3. In some cases the speed of reaction was very high, and practical cements could not be formed from ZnO or CaO even when these oxides were deactivated by heating. All the faster-setting cements exhibited good hydrolytic stability. The stability of the complexes between divalent cations and PVPA was found by a titrametric procedure to follow the order Mg Ca < Cu Zn (Ellis Wilson, 1991). This result was... 

Triphenylformazan behaves as a bidentate ligand forming 2 1 complexes (217) with divalent copper, nickel, and cobalt.377 Formazan metal complexes can be compared to complexes of azo dyes or beta diketones due to structural similarity.301,302 In general, formazan metal complexes have low stability toward acids. However, when electron-donating substituents are added to the aromatic ring, a considerable enhancement in stability is observed. Cationic complexes of type 218 are also known. The complexation of formazan with metal cation can be accompanied by oxidation to the tetrazolium salt and the formation of a complex... 

A number of substances have been discovered in the last thirty years with a macrocyclic structure (i.e. with ten or more ring members), polar ring interior and non-polar exterior. These substances form complexes with univalent (sometimes divalent) cations, especially with alkali metal ions, with a stability that is very dependent on the individual ionic sort. They mediate transport of ions through the lipid membranes of cells and cell organelles, whence the origin of the term ion-carrier (ionophore). They ion-specifically uncouple oxidative phosphorylation in mitochondria, which led to their discovery in the 1950s. This property is also connected with their antibiotic action. Furthermore, they produce a membrane potential on both thin lipid and thick membranes. 

It needs to be noted that when the ligand system contains extensive unsaturation, then oxidation of the corresponding complex may yield a product containing a stabilized cation radical (rather than one in which the metal oxidation state has been altered). For example, such a situation has a tendency to occur on oxidation of divalent metal complexes [including Ni(n)] of the tetraphenyl-substituted porphyrin macrocycle. 

Solid solutions within the apatite family are readily synthesized in the laboratory. Some examples include (Ca,Zn,Pb)5(P04)30H (Panda et al. 1991), (Ca,Cd,Pb)5(P04)30H (Mahapatra et al. 1995), (Ca,Sr,Cu)5(P04)30H (Pujari Patel 1989), or other apatite solid solutions containing various quantities of Cd, Mg, Zn, Cd or Y (Ergun etal. 2001). They are also found naturally (Botto etal. 1997). Unlike well-ordered naturally occurring minerals, these solid solutions may actually be the more typical form of the mineral in stabilized ash systems given the system complexity, rapid precipitation kinetics, and wide prevalence of available divalent cations. 

This [or, more accurately, Eq (1.35)] allows us to calculate values of the stability constant of a complexant from tables of electrochemical potentials. Eor example, a shift of 300 mV in potential due to complexation gives an (approximate) value for the stability constant of that complex of 10 (for n= 1) or, for the more common case in CD, where divalent cations (n = 2) predominate, 10 °. 

R. N. Smith and R. A. Alberty, The apparent stability constants of ionic complexes of various adenosine phosphates and divalent cations, J. Am. Chem. Soc., 78. 2376-2380 (1956). 

Crystal field spectral measurements of transition metal ions doped in a variety of silicate glass compositions (e.g., Fox et al., 1982 Nelson et al., 1983 Nelson and White, 1986 Calas and Petiau, 1983 Keppler, 1992) have produced estimates of the crystal field splitting and stabilization energy parameters for several of the transition metal ions, examples of which are summarized in table 8.1. Comparisons with CFSE data for each transition metal ion in octahedral sites in periclase, MgO (divalent cations) and corundum, A1203 (trivalent cations) and hydrated complexes show that CFSE differences between crystal and glass (e.g., basaltic melt) structures,... 

In the absence of such data, plots of stability constants for isostructural metal complexes formed with a common ligand against EN values for the metal ions are still useful. As shown in Figure 2, the stability of divalent metal complexes with HCO3 and S04 is practically independent of cation electronegativity, whereas the metal complexes formed with HP04 and 003 are increasingly dependent on EN, particularly for EN >1.5. Based on the relative solubilities of their salts, EN values for C0 2- ... 

Comparison plots such as these are a valuable predictive tool. Such plots are most useful when the cations or ligands chosen for comparison have similar properties, such as equal valence, similar size and geometry, and similar electron configurations and bonding properties. In this way, trends or differences in their plotted behavior can be more rationally understood. For example. Figure 6 compares the stability constants of oxalate and carbonate complexes with the same divalent cations. The equation of the line is log q = 1.11 log o- The plot... 

Beryllium forms an enolate complex with lapachol (11a), a versatile ligand that also forms complexes with a variety of other divalent cations, e.g. Pd(ll) and ZrO(II), and trivalent metallic cations , e.g. In(lll) and Rh(lll), for which there is quantitative stability data. There are no data on related saturated derivatives, e.g. the analogous species 11b, to assess additional stabilization due to jt-metal interactions. 

The potential for RNA to act as a catalyst is dictated by its structure as a linear polymer of the four common ribonucleotides. Like DNA, RNA can form double stranded, antiparallel helices via traditional Watson-Crick base pairing. However, the backbone of nucleic acid is highly flexible and RNA can form complex tertiary structures that often involve non-Watson-Crick base pairing to create active site crevices for catalysis. The phosphodiester backbone is charged negatively and interacts electrostatically as well as by direct coordination with solution divalent cations. Ribose, purines, and pyrimidine bases contain both H-bond donors and acceptors that help stabilize higher-order stmcture and provide for substrate positioning, as well as participate in active site interactions. 

Turner et al. (1981) and Byrne et al. (1988). Turner et al. (1981) used a database of stability constants for more than 500 metal complexes to calculate the inorganic speciation for 58 trace elements in model seawater at pH 8.2, 25 °C, and 1 atm. Byrne et al. (1988) extended this work by considering the influence of temperature and pH on speciation. The free hydrated divalent cation dominates the dissolved inorganic speciation of Zn(II) and the first transition series metals Mn(II), Co(II), and Ni(II). Strongly hydrolyzed trace metals include Be(II), Al(III), Fe(III),... 

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