Factors that influence complex stability

The term stable complex is defined in terms of the equilibrium eonstant for the formation of the complex. The equilibrium constant of a reaction is a measure of heat released in the reaetion and the entropy ehange during the reaetion. The greater the amount of heat evolved in a reaction the more stable are the reaetion products. The entropy of a system is a measure of the amount of disorder. The greater the amount of disorder in the products of a reaction relative to the reactants, the greater the increase in entropy during the reaction 

Relative stabilities of many complexes can be understood in terms of a simple electrostatic model. The predictions of this model are related primarily to the heat evolved in the formation of the complexes. We are familiar with the observation that oppositely charged particles attract one another, whereas like charged particles repel one another. Moreover, the repulsion or attraction depends upon the distance between the centers of the particles, being greater as the particles approach one another. 

One would, therefore, predict that the most stable complexes would be made up of oppositely charged ions and, moreover, that the greater the charge on the ions and the smalller the ions the greater should be the stability. Small ions are favored because their centers can be closer together. From this point of view the stability of complexes should increase with the charge on metal ions. One illustration of this behavior is the increase of stability of hydroxide complexes with an increase in charge of the metal ion. The stability of 

The stabilities of high-spin complexes of the ions between Mn and Zn with a given ligand frequently vary in the order Mn Fe Co Ni Cu Zn. This order, sometimes ealled the natural order of stability, is relatively consistent with the charge-to-radius concept, sinee the radii of the ions vary in the order Mn Fe Co Ni Cu Zn. Both the variation in size of the cation and the order of stability ean be explained in terms of the crystal field stabilization energy (CFSE) for these complexes (Section 2.5). 

It has been observed that the greater the base strength of a ligand, the greater is the tendency of the ligand to form stable metal complexes. The base strength of a molecule is a measure of the stability of the complex that the molecule forms with H. It is reasonable that ligands which bind firmly should also form stable complexes with metal ions. From this point of view, F 

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Ligands that interact physically with DNA have been extensively studied both by experimental techniques and by a variety of theoretical approaches. A diverse set of compounds have been studied, including compounds that intercalate between DNA sequences or bind in the minor groove.1 7 These studies have identified various factors that influence the stability of DNA ligand complexes in solution.6 8 9... 

Some factors that influence the stability of polymer chelates should be mentioned. Hojo et a/.61) have reported the effect of the ligand ratio [ligand]/[metal ion] on the formation of the Cu chelate of poly(vinylalcohol)(PVA). Figure 10 shows the relationship between the formation constant of the Cu complex, the viscosity of an aqueous solution of PVA, and the ligand ratio. The viscosity diminishes very sharply at about [PVA]/[Cu] = 32 this corresponds to an increase in the formation constant. A tightly packed conformation of PVA, caused by intra-polymer chelation with Cu, facilitates more and more chelate formation. 

Platinum (II) complexes of the type (COD)PtRX (COD = jj -l,5-cyclooctadiene R, X = alkyl, aryl, halide) are known for many combinations of R and X, and then-reaction chemistry is well developed see Platinum Organometallic Chemistr. The lability of the COD-Pt interaction renders these compounds convenient sources of organoplatinum fragments for coordination to Lewis bases such as phosphines, according to equation (27). In an effort to better understand the factors that influence the stability of bis(phosphine)platinum(II) species, a thermodynamical study of the reaction in equation (27) was undertaken for a series of monodentate and bidentate phosphine ligands of varying steric and electronic character. 

There is a lot of information available on the stabilities of metal complexes. This permits an assessment of the various factors that influence the stability of a metal complex. Some of these factors have already been discussed, but it may be helpfiil to summarize them briefly. First, the stability of a complex obviously depends on the nature of the metal and of the ligand. With reference to the metal, the following factors are important ... 

The purpose of this paper is to describe the principal factors that influence the stabilities of the complexes formed in solutioa This involves heats and entropies of complex formation and the factors which favor a more negative enthalpy and a more positive entropy. Also, a number of misconceptions that have been adopted in foe recent literature are described and replaced by a more reasonable explanation of foe chelating tendencies involved. 

While this feedback may or may not be climatically relevant, it does serve to illustrate the nature of biogeochemical feedbacks. It seems likely that many such complex systems exist, and that they may indeed be factors that influence climate. To return to the introduction to this chapter, it is not possible to rule out biogeochemical feedbacks as factors that have stabilized climate over the past ca. 10 years. 

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. 

When both rings in azapentalenes have more than one nitrogen atom, the CH tautomer is unstable thus structure 27Sa, proposed by some workers,148 is unlikely, and the compound is better represented by the NH forms 275b or 275c. The factors that influence the relative stabilities of NH tautomers are more complex and can be summarized in four rules ... 

Structural effects on the C-basicity of enamines are, however, more complex. Because of the paucity of values for pX H+, we shall anticipate our discussion of the kinetics of C-protonation (see Section III) so that some of the information presented there can be incorporated into the present section. The justification for doing so is that many of the effects that influence the stability of the iminium ion are expected to be operational in the transition state. In particular, the coplanarity of the atoms about the C=N bond in the iminium ion (already preferred for some enamines, but only when geometrically possible24) should be maintained or improved in the transition state in order to maximize p-n overlap (equation 4)25. This means that, besides the ability of the amino nitrogen to bear a positive charge, other factors such as formation of the C=N double bond (with attendant rehybridization at nitrogen), and steric interactions between groups attached to the alkene and amine moieties will be important both in the transition state and in the iminium ion product. 

In addition to enantiocontrol, the problem of regiocontrol arises in these reactions. There are various factors that influence the regioselectivity of allylic substitutions [3,4,13, 36, 37, 38, 39]. Electronic effects exerted by the catalyst and the allylic substituents, steric interactions between the nucleophile, the allyl system and the catalyst, and the relative stabilities of the Ti-olefin complexes formed after nucleophilic addition, can all play a role. The relative importance of these factors varies with the catalyst, the substrate, the nucleophile, the solvent and other reaction parameters and is difficult to predict. 

The diverse applications of complex polysaccharide/protein offer new tools to improve the quality of texture and stability of food products, cosmetics, and pharmaceuticals, plus the ability to generate new functional structures at the micro- and the nanoscale [4, 80]. More studies should be conducted on the factors that influence and affect the structures of protein-polysaccharide complexes to better understand the associative interactions of mixtures and achieve full potential as stabilizers, foaming, emulsifying, and haulers as matrices. 

Electronic factors also influenced the outcomes of these cyclization reactions cyclization of pyrrole 84 to bicyclic amine 85 is catalyzed by the sterically open complex 79a. In this reaction, initial insertion into the Y - H bond occurred in a Markovnikov fashion at the more hindered olefin (Scheme 19) [48]. The authors proposed that the Lewis basic aromatic ring stabilizes the electrophilic catalyst during the hydrometallation step, overriding steric factors. In the case of pyrroles and indenes, the less Lewis basic nitrogen contained in the aromatic systems allowed for the cyclization of 1,1-disubstituted alkenes. 

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