Factors influencing the stability of complexes

In the above equilibria it has been assumed that no insoluble products are formed nor any polynuclear species. 

A knowledge of stability constant values is of considerable importance in analytical chemistry, since they provide information about the concentrations of the various complexes formed by a metal in specified equilibrium mixtures this is invaluable in the study of complexometry, and of various analytical separation procedures such as solvent extraction, ion exchange, and chromatography.2,3 

This shows that the pM value of the solution is fixed by the value of K and the ratio of complex-ion concentration to that of the free ligand. If more of M is added to the solution, more complex will be formed and the value of pM will not change appreciably. Likewise, if M is removed from the solution by some reaction, some of the complex will dissociate to restore the value of pM. This recalls the behaviour of buffer solutions encountered with acids and bases (Section 2.20), and by analogy, the complex-ligand system may be termed a metal ion buffer. 

The stability of a complex will obviously be related to (a) the complexing ability of the metal ion involved, and (b) characteristics of the ligand, and it is important to examine these factors briefly. 

Cations with noble gas configurations. The alkali metals, alkaline earths and aluminium belong to this group which exhibit Class A acceptor properties. Electrostatic forces predominate in complex formation, so interactions 

From a more general standpoint and from the viewpoint of the solution chemistry, this theory appears to be an attempt to reunify the reactions that may occur in aqueous solutions  

Finally, it brings nothing new, but it may perhaps permit us to reason easily at the time of the study of complexes formations by making use of the reasoning already followed to study acid-base and redox phenomena. However, this theory appears to be artificial, since there are great differences in the different particles behaviors  

A well-defined species of the formula [Cl(H20)m] , in which true strong bonds between the chloride ion and the water molecules would exist, is unknown. This half-equilibrium corresponds more to a solvation process than to a true chemical 

Complexes may be more or less stable, more or less perfect. Now, it is interesting to find the factors that govern the ligands reactivities toward the central metallic ions and those that govern the stability of complexes. The ability to complex metallic ions and also ligands is now considered. (The external factors playing a part in the complexes formation, such as the superimposition of varied equilibria, are investigated only in the next chapter.) 

In terms of the ability of metallic ions to form complexes, Schwarzenbach distinguished two categories classes A and B. 

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Factors Influencing the Stability of Complexes Table 24.2 Hardness and softness of some metallic ions... 

Section III.C A Hydrido(methyl)carbene Complex of Platinum(IV) (223) and Methyl(hydrido)platinum(IV) Complexes with Flexible Tridentate Nitrogen-Donor Ligands (224) are structurally related to the system shown in Scheme 13 and give additional information on how steric and electronic factors influence the stability of platinum(IV) methyl hydrides. 

The stability constant of complexes between /1-cyclodextrine and p-nitroaniline is higher than that of aniline because the resonace charge delocalization (and London dispersion interactions) is an important factor influencing the stability of these complexes62. This behaviour parallels that of corresponding phenols. 

Factors Influencing the Stability of "Unsupported" Metal-Metal Bonds in Ti/Zr/Hf-M Heterodimetallic Complexes... 

The factors which influence the stability of metal ion complexes have been discussed in Section 2.23, but it is appropriate to emphasise here the significance of the chelate effect and to list the features of the ligand which affect chelate formation ... 

Other factors influencing the rate of metal-ion transport across artificial membranes have been identified. As might be expected, such transport is dependent on the interplay of several factors. For example, as briefly mentioned already in Chapter 4, it is clear that the strength of complex-ation of the cation by the carrier must be neither too high nor too low if efficient transport is to be achieved. If the stability is low, then uptake of the metal ion from the source phase will be inhibited. Conversely, for those cases where highly stable complexes are formed, there will be a reluctance by the carrier to release the cation into the receiving phase. 

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... 

Stephenson monitored the quenching of aromatic hydrocarbon singlet states by observing a decrease in the sensitizer fluorescence intensity with added diene.158 A marked effect of the structure of both the sensitizer and the quencher was noted, with values of kq ranging from 4 x 109 to 8.7 x 10s however, no quantitative correlation between k and several factors that might influence the stability of the proposed excited complex could be obtained (see below). The important observation that singlet quenching led to... 

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. 

The formation of complexes is affected by many physical and chemical factors. Such environmental factors as solvent, temperature and pressure are often important. Concentration factors sometimes markedly influence the stabilities of the complexed species in solution. The role of the donor atoms of the ligand in forming complexes has already been mentioned. 

While the example illustrated in Scheme 5.8 shows equilibrium between two chemically identical carbocations, there are factors influencing the direction of these transformations when applied to more complex systems. If we consider Scheme 5.9, we notice that the positive charge migrates exclusively to the tertiary center, reflecting the increased stability of tertiary carbocations over primary carbocations. In general, where 1,2-hydride shifts are possible, rearrangement of less stable carbocations to more stable carbocations is expected. 

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. 

A question must be raised as to why the tap water and the calcium bicarbonate washing treatments give such different results. The alkaline pH of the tap water suggests that calcium carbonate or bicarbonate is present and that the final product deposited in the fiber should be similar to that obtained with the pure bicarbonate solution. However, the chemical makeup of any city tap water is very complex and must contain a number of components that could affect the stability of cellulose. For example, the municipal treatment plant in Ottawa adds large amounts of alum (aluminum sulfate) to the water to settle particulate matter. Because alum makes the water very acidic, lime is then added to raise the pH. The result is that a large amount of calcium sulfate is present in the tap water and must affect the overall chemistry of the salts deposited in the fibers. One may further speculate that the anions present can influence the stability of cellulose as much as the cations. Any comprehensive understanding of the factors involved must include aH parameters. 

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