Equilibrium chelate effect

MePO2- or PME2- (Table XIX), but the open closed equilibrium lies very much on the side of the chelated form of the complex (87% for the Ca2+ complex - compare 15% for [Ca(atp)]2 and just 7% for [Ca(amp)] (695)). The availability of stability constants both for methylphosphonate and for benzimidazole (a purine model) complexes means that the chelate effect for complexes of (1H-benzimidazol-2-yl-methyl)phosphonate can be discussed without the usual complications, such as the differences between ethane-1,2-diamine and two ammonia or two methylamine ligands and disparities between units (704). 

Selected entries from Methods in Enzymology [vol, page(s)] Buffer capacity, 63, 4 choice, 63, 19, 20, 285 metal ion chelation effects, 63, 225, 226, 287, 298, 299 dielectric constant effect on pK, 63, 226 dilution, 63, 20 equilibrium constant effects, 63, 18 heavy water, 63, 226, 227 ionic strength effects, 63, 226,... 

Major advances and problems in the field of synthesis, properties, structure and applications of polymers containing metallochelate units are discussed. Included are terminology, classification and nomenclature of these compounds as well as major approaches to calculating the equilibrium constants of chelation with polymeric ligands and chelate effect in metallopolymeric systems. Special attention is paid to the production and structural features of polymers containing metallochelate units. The most important applications of such polymers are classified... 

The term chelate effect refers to the enhanced stability of a complex system containing chelate rings as compared to the stability of a system that is as similar as possible but contains none or fewer rings. As an example, consider the following equilibrium constants ... 

Hydration enthalpy Stability or formation constant Overall and stepwise stability constants Chelate effect Macrocyclic effect Preorganization Equilibrium template effect Kinetic template effect Self-assembly... 

Many other examples of steric effects are compiled in recent equilibrium data (17) j and many of the observed effects are apparently due to enthalpies and entropies of formation of the free ligand in solution. For this reason, more reliable data on heats and entropies of chelate formation and on heats and entropies of formation of metal ions and ligands in solution should be accumulated for developing further understanding of the chelate effect. 

Threo diastereoselectivity is consistent with a chelation-controlled (Cram cyclic model) organolithium addition (Figure 8a). Since five-membered chelation of lithium is tenuous, an alternative six-membered chelate involving the dimethylamino nitrogen atom of the thermodynamically less stable (Z)-hydrazone (in equilibrium with the ( )-isomer) cannot be discounted. The trityl ether (entry 4, Table 9) eliminates the chelation effect of the oxygen atom such that the erythro diastereomer predominates (via normal Felkin-Ahn addition) (Figure 8b). 

The thermodynamic or equilibrium template effect is based on the fact that in the absence of the template the desired product is formed, but with one or more (undesired) compounds that are in equilibrium with each other [42], A marked tendency of the product to form chelates enables a template to displace the product equilibrium in the desired direction. The first example of an equilibrium template effect was the formation of a nickel(II) complex of the Schiff base 33 formed from -aminoethanethiol 31 and the a-diketone 32 (Scheme 11) [43]. 

Intramolecular reactions are often more favored than analogous intermole-cular reactions. This advantage is known either as the proximity effect or as the chelate effect. To set it on a quantitative scale, the effective molarity (EM) parameter has been devised [54-56]. For thermodynamically controlled processes, the EM is defined by Eq. 11, where iCintra and iCinter are the equilibrium constants for the intramolecular process leading to a cyclic species and the corresponding intermolecular process leading to a linear adduct (Fig. 9). 

Comparison of the complex-formation constants for bofli 1 1 (57 and 58) and 1 2 (such as 59) species ° with those obtained for the respective copper(II) complexes with parent amino acids revealed that the fructosyl moiety provides for an additional chelate effect in D-fructose-a-amino acids and as a consequence, a significant increase in the complex stability. In the absence of an anchoring chelating group, such as a-carboxylate, the D-finctosamine structure is not a good copper(II) chelator, and Cu(n) expectably does not form stable complexes with the carbohydrate in A -d-Iructose-L-lysine peptides. Although it would be expected that iron(III) complexes with D-finctose-amino acids in aqueous solutions, no related thermodynamic equilibrium studies have been done so far for this important redox-active metal. 

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