Stability of complex ions

The stability of complex ions varies within very wide limits. It is quantitatively expressed by means of the stability constant. The more stable the complex, the greater is the stability constant, i.e. the smaller is the tendency of the complex ion to dissociate into its constituent ions. When the complex ion is very stable, e.g. the hexacyanoferrate(II) ion [Fe(CN)6]4", the ordinary ionic reactions of the components are not shown. 

Table 11. STABILITY OF COMPLEX IONS IN AQUEOUS SOLUTIONS...

The logical place to begin a discussion of the formation and stability of complex ions in aqueous solutions is with the aqueous ions themselves. If we regard the (metal) ion as being an aqueous complex (M(H20) )i , which is then further and more loosely solvated, we wish to know the coordination number n and also the manner in which the n molecules are arranged around the metal ion. 

Excluded here are those techniques relating to tracer-level work (below weighable quantity of sample measurement by radioassay only), for the concern here will be with the determination of bulk properties of these elements. Tracer-scale studies usually reveal directly only one property of the element under investigation, that is, its relative preference for one environment over another, or more simply, its phase distribution. Nevertheless, as each new transuranium element was discovered and was available only in trace quantities, a great deal of chemistry was learned by inference from tracer-scale studies, including the identity of oxidation states, approximate values of oxidation or reduction potentials, the composition and stability of complex ions, and relative volatilities. 

In a fundamental sense metal ions simply dissolved in water are already complexed—they have formed aquo ions. The process of forming what we more conventionally call complexes is really one of displacing one set of ligands, which happen to be water molecules, by another set. Thus the logical place to begin a discussion of the formation and stability of complex ions in aqueous solution is with the aquo ions themselves. 

A method has been developed for calculating stabilities of complex ions in binary molten salt mixtures, using accurate enthalpy of mixing data as bases. It was possible to discriminate between two possible models for the dissociation of molten cryolite ... 

The lanthanide contraction well exemplifies a gradation in the chemical properties, such as decreasing basicity, ionic characteristics, solubility (the solubility products of La(OH>3 and Lu(OH)3 are 1.7 X 10 and 1.9 X lO, respectively [7]) and stability of complex ions. Obviously, as the chemistry of the lanthanides is so similar their isolation in highly pure forms was difficult until the advent of ion exchange chromatography and liquid-liquid extraction procedures. 

Rejerences. — For solubility determinations Goodwin, Zeitsckr. physikal. Chan., 1894, 18, 577 Ab and Cox, ibid, 1903, 46, i. For stability of complex ions Bod nder and Fittig, Zeittehr. fhysikal. Chan., 1902,39,597. For determinations of hydrolysis of salts Denham, Trans. Chan. Soc., 1908, 93, 41. For decomposition potentials Bennewits, Zeitschr, yhysikal. Chan., 1910, 72, 202. 

The terms labile and inert are not related to the thermodynamic stabilities of complex ions or to the equilibrium constants for ligand-substitution reactions. The terms are kinetic terms, referring to the rates at which ligands are exchanged. 

The stability of tin over the middle pH range (approximately 3-5-9), its solubility in acids or alkalis (modified by the high hydrogen overpotential), and the formation of complex ions are the basis of its general corrosion behaviour. Other properties which have influenced the selection of tin for particular purposes are the non-toxicity of tin salts and the absence of catalytic promotion of oxidation processes that may cause changes in oils or other neutral media affecting their quality or producing corrosive acids. 

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 ... 

TABLE 11.1 Relative Stabilities of Arenium Ions and ir Complexes and Relative Rates of Chlorination and Nitration ... 

Ahrland et al. (1958) classified a number of Lewis acids as of (a) or (b) type based on the relative affinities for various ions of the ligand atoms. The sequence of stability of complexes is different for classes (a) and (b). With acceptor metal ions of class (a), the affinities of the halide ions lie in the sequence F > Cl > Br > I , whereas with class (b), the sequence is F < Cl" < Br < I . Pearson (1963, 1968) classified acids and bases as hard (class (a)), soft (class (b)) and borderline (Table 1.23). Class (a) acids prefer to link with hard bases, whereas class (b) acids prefer soft bases. Yamada and Tanaka (1975) proposed a softness parameter of metal ions, on the basis of the parameters En (electron donor constant) and H (basicity constant) given by Edwards (1954) (Table 1.24). The softness parameter a is given by a/ a - - P), where a and p are constants characteristic of metal ions. 

It is important to recognize some of the limitations of the Pourbaix diagrams. One factor which has an important bearing on the thermodynamics of metal ions in aqueous solutions is the presence of complex ions. For example, in ammoniacal solutions, nickel, cobalt, and copper are present as complex ions which are characterized by their different stabilities from hydrated ions. Thus, the potential-pH diagrams for simple metal-water systems are not directly applicable in these cases. The Pourbaix diagrams relate to 25 °C but, as is known, it is often necessary to implement operation at elevated temperatures to improve reaction rates, and at elevated temperatures used in practice the thermodynamic equilibria calculated at 25 °C are no longer valid. 

Stability of Complexes of Imidazole with Metal Ions of Biological and Biomedical Interest ... 

The stability of metal ion-alkane adducts such as shown in Figure 11 remains an interesting question. The bonding in such systems can be regarded as intermolecular "agostic" interactions (46). Similar adducts between metal atoms and alkanes have been identified in low-temperature matrices (47). In addition, weakly associated complexes of methane and ethane with Pd and Pt atoms are calculated to be bound by approximately 4 kcal/mol (43). The interaction of an alkane with an ionic metal center may be characterized by a deeper well than in the case of a neutral species, in part due to the ion-polarization interaction. 

The decrease in size of the +3 ions of the lanthanides is also a factor in stability of complexes of these ions, and complexes with a given ligand usually show an increase in stability in progressing through... 

Although the subject of stability of complexes will be discussed in greater detail in Chapter 19 it is appropriate to note here some of the general characteristics of the metal-ligand bond. One of the most relevant principles in this consideration is the hard-soft interaction principle. Metal-ligand bonds are acid-base interactions in the Lewis sense, so the principles discussed in Sections 9.6 and 9.8 apply to these interactions. Soft electron donors in which the donor atom is sulfur or phosphorus form more stable complexes with soft metal ions such as Pt2+ or Ag+, or with metal atoms. Hard electron donors such as H20, NH3( or F generally form stable complexes with hard metal ions like Cr3+ or Co3+. 

It is possible to determine the concentration of certain metal ions by performing a titration in which the complexation of the metal is the essential reaction. Typically, a chelating agent such as EDTA is used because the complexes formed are so stable. The specific composition of complexes formed in solutions often depends on the concentrations of the reactants. As a part of the study of the chemistry of coordination compounds, some attention must be given to the systematic treatment of topics related to the composition and stability of complexes in solution. This chapter is devoted to these topics. 

The formation of a coordinate bond is the result of the donation and acceptance of a pair of electrons. This in itself suggests that if a specific electron donor interacts with a series of metal ions (electron acceptors) there will be some variation in the stability of the coordinate bonds depending on the acidity of the metal ion. Conversely, if a specific metal ion is considered, there will be a difference in stability of the complexes formed with a series of electron pair donors (ligands). In fact, there are several factors that affect the stability of complexes formed between metal ions and ligands, and some of them will now be described. 

Establishing correlations between the stability of complexes and factors related to the characteristics of the metal ion and ligands involved is not a new endeavor. One of the earliest correlations established showed that for many types of ligands, the stability of the complexes that they form with +2 ions of first-row transition metals varies in the order... 

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