Metal complex ions, formation constants

In common with other hydroxy organic acids, tartaric acid complexes many metal ions. Formation constants for tartaric acid chelates with various metal ions are as follows Ca, 2.9 Cu, 3.2 Mg, 1.4 and Zn, 2.7 (68). In aqueous solution, tartaric acid can be mildly corrosive toward carbon steels, but under normal conditions it is noncorrosive to stainless steels (Table 9) (27). 

A measure of the tendency of a metal ion to form a particular complex ion is given by the formation constant Kf (also called the stability constant), which is the equilibrium constant for the complex ion formation. The larger Kf is, the more stable the complex ion is. Table 16.4 lists the formation constants of a number of complex ions. 

Complex ions are formed in solution by the combination of a metal cation with a Lewis base. The formation constant Kf measures the tendency toward the formation of a specific complex ion. Complex ion formation can increase the solubility of an insoluble substance. 

So pronounced is the chelating tendency of the diketonate anion that even alkali metal complexes may be isolated, as illustrated by RbiKCFjCOCHCOCFjljNa], in which sodium is surrounded by a trigonal prismatic array of donor oxygen atoms. For complexes derived from dibenzoylmetbane, stabilities in dioxane-water are in the order Li > Na > K > Cs. For divalent metal ions formation constants increase in the order Ba > Sr > Ca > Mg > Cd > Mn > Pb > Zn > Co, Ni, Fe > Cu, and for higher valent metal ions the first formation constants for chelates are in the order Fe + > Ga " " > Th" > In " " > Sc " " > Y " " > Sm " > Nd " > More recently, such stu-... 

A reaction scheme that illustrates the role of the metal ion in the enzyme activity can be shown by the following reactions, in which E represents the enzyme, S is the substrate, M is the metal ion, P is the product, and EM is the enzyme-metal complex. The rate constant for the formation of the enzyme-metal complex is and that for its dissociation is k-. ... 

Some polystyrene resins (cross-linked with DVB) are specially modified to have chelating functional groups bound to the matrix so as to make them selective towards certain ions. Such resins with iminodiacetic acid groups are marketed under the trade names Dowex A-1 (Dow Chemical) and Chelex 100 (Bio-Rad Laboratories). The complex (XXVI) formation constants with metal ions of the chelating resin are so large that the resin absorbs metal ions equivalent to the iminodiacetic acid groups (used in sodium salt form), i.e., the efficiency of metal ion adsorption is near 100%. A particular metal ion can be removed by controlling the pH of aqueous solution. For example, at pH 2, mercury and copper ions are... 

TABLE 17.1 Formation Constants for Some Metal Complex Ions in Water at 25 "C ... 

The solubility of metal salts is also affected by the presence of certain Lewis bases that react with metal ions to form stable complex ions. Complex-ion formation in aqueous solution involves the displacement by Lewis bases (such as NH3 and CN ) of water molecules attached to the metal ion. The extent to which such complex formation occurs is expressed quantitatively by the formation constant for the complex ion. Amphoteric oxides and hydroxides are those that are only slightly soluble in water but dissolve on addition of either acid or hase. 

Another difference lies in the polyfimctional nature of metal ions. They are capable of reacting with more than one ligand, forming a series of complexes whose equilibrium constants are much more closely spaced than those of most polyprotic acids. For example (Figure 9.1), the titration of Cu with NH3 would certainly be of dubious practical analytical value. Copper(II) ion forms a series of ammonia complexes whose formation constants are both small and closely spaced there are several complex species present in significant concentrations throughout the titration. 

A metal ion combined with a Lewis base, such as Ag(NHj>2 (aq), is called a complex ion. The equilibrium constant for the formation of a complex ion from a metal ion in aqueous solution is called the formation constant (A ) of the complex ion. The higher the value of Af the more stable is the complex ion. Values of the formation constants for some metal complex ions are given in Appendix IV. 

The concentration of a metal ion in solution is decreased by complex-ion formation. The equilibrium constant for the formation of the complex ion from the aqueous metal ion and the ligands is called the formation constant (or stability constant), Kf. Because complex-ion formation reduces the concentration of aqueous metal ion, an ionic compound of the metal is more soluble in a solution of the ligand. 

The formation constants of EDTA complexes are gathered in Table 11.34. Based on their stability, the EDTA complexes of the most common metal ions may be roughly divided into three groups ... 

The formation constant for a metal—ligand complex in which only one ligand is added to the metal ion or to a metal—ligand complex Ki). 

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