Acid dissociation constants complex ions

To conduct meaningful mechanistic and kinetic studies in alcohol media reliable and simple measurement and control of the solution jjpH is essential. Potentiometric titration is the method of choice for obtaining acid dissociation constants or metal ion complex stability constants and in favorable cases the speciation of mixtures of metal-ion-containing complexes in solution can be proposed.20 Titrations in non-aqueous solvents are not nearly as widely reported as those in aqueous media, particularly in cases with metal ions21 and determination of pH in a non-aqueous solvent referenced to that solvent is complicated due to the lack of a way to relate the electrode EMF readings to absolute jjpH (see footnote and ref. 6) so non-aqueous solvents are generally inconvenient to use22 for detailed studies of reaction mechanisms where pH control is required. 

Metal-complex stability is also related to the basic strength of the ligand entity. For a series of 1 2 complexes of the bidentate naphthylazophenol ligand (5.64) with copper(II) ion, the acidic dissociation constants (pKa) are linearly related to the stability constants (log K1 2), the more acidic groups forming the less stable complexes. Thus where X = N02 in structure 5.64 then pKa = 8.1 and log K1 2 = 17.2, and where X = OCH3 then pKa = 8.5... 

In a similar investigation of the tautomeric tridentate ligand 2 -hydroxyphenylazo-2-naphthol (5.65 in Scheme 5.17), the first and second acidic dissociation constants (pKa) related to the two hydroxy groups in the parent structure (X = H) were found to be 11.0 and 13.75 respectively. On introduction of an electron-withdrawing substituent (X) the first dissociation constant decreased from 11.0 to 10.55 (X = Cl) or 7.67 (X = N02). The stability constants (log K1 1) of the derived 1 1 complexes were dependent on the metal ion introduced [46], being particularly high for nickel(n) at 19.6 and copper(II) at 23.3. 

The neutral carboxyl group is not very effective in increasing the reduction rate of the complex. However, when the proton is removed from the carboxyl, the effect can increase and is greatest when the carboxyl ion is in a configuration favorable to chelation. Thus, the inverse (H+) path is not even observable for acid succinate in the same acidity range as that for which this path is important in the acid malonato reaction. The acid dissociation constants are known well enough so that the behavior difference between acid malonato and acid succinato can not be entirely ascribed to different acidities of the complexes. The results obtained with the acid malonate complexes, as reported in Table II, incidentally provide no support for the hypothesis (22) that electron transfer takes place by remote attack across hydrogen bonds. 

Gallium hydroxide is amphoteric, and is a much stronger acid than aluminum hydroxide. For Ga(OH)3 the first acid dissociation constant is 1.4 x 10-7 [for Al(OH)3 the value is 2 x 10-11].1 Polymerization occurs in aqueous Ga3+ solutions to which OH- is added,533 but this tendency is less than in the case of aluminum solutions (Section 25.1.5.1). The formation constants of mononuclear hydroxo complexes of Ga, including Ga(OH)4, and the hydrolysis constants of gallium ions have been measured by a competing ligand technique.534... 

Condensation to monohydroxo-bridged complexes is often described by Eq. (28), for which the equilibrium constants Kd are related to those defined by Eq. (27) by Kd = Q2i/Qh, where is the first acid dissociation constant of the mononuclear aqua ion. 

Acid Dissociation Constants and Dissociation Constants of Complex Ions... 

In using acid dissociation constants and the dissociation constants of complex ions, it is convenient to take the base 10 logarithms of equations 1.2-7 and 1.2-9 to obtain... 

Acid dissociation constants and dissociation constants of complex ions determine the concentrations of species that are present in a solution at equilibrium under specified conditions. Ionic dissociation reactions occur rapidly and tend to remain at equilibrium during an enzyme-catalyzed reaction. Since ATP (see Fig. 1.1) is the primary carrier of energy in biochemical systems and since a good deal is known about its binding properties, these properties are considered here in some detail. 

In this chapter we have seen that acid dissociation constants are needed to calculate the dependence of apparent equilibrium constants on pH. In Chapter 3 we will discuss the calculation of the effects of ionic strength and temperature on acid dissociation constants. The database described later can be used to calculate pKs of reactants at 298.15 K at desired ionic strengths. Because of the importance of pKs of weak acids, Table 1.3 is provided here. More experimental measurements of acid dissociation constants and dissociation constants of complex ions with metal ions are needed because they are essential for the interpretation of experimental equilibrium constants and heats of reactions. A major database of acid dissociation constants and dissociation constants of metal ion complexes is provided by Martell, Smith, and Motekaitis (2001). 

As a preliminary to a discussion of kinetics of reactions in aqueous mixtures, it is interesting to review briefly the behaviour of equilibrium quantities as a function of co-solvent mole fraction. Interpretation of the data is necessarily complex because, for example, in the case of acid dissociation constants, the quantity 5mAXie represents the result of the individual variations of the partial molar quantities for acid, conjugate base and hydrogen ion. Nevertheless patterns of behaviour are observed which demonstrate the impact of co-solvent on water structure and on solute properties along the lines discussed in the previous section. 

The effect of metal chelation on the acidity of 4-(2-pyridylazo)-resorcinol (IX) has also been investigated by Freiser 18y 19), The acid dissociation constants for the ligand were found to be pKa = 7.0, pKa = 12.4. Upon coordination to a metal ion, the proton on the 3-hydroxy group is displaced so that chelation may occur. The remaining 1-hydroxy group is acidic, and the acid dissociation constants for complexes of some divalent metal ions are shown in Table IV. 

Most metal ions effectively reduce the pKa (— log of the acid dissociation constant, K,A) of a solvent water molecule, so knowing the pKa values of metal aquo complexes can be useful. Some values are given in Table 1.5. 

Distorted octahedral structures have been proposed for the 1 1 neutral complexes formed between bivalent transition-metal ions and hydroxymethyl-phosphonic acid, H0CH2P03H2. A series of new dialkylaminomethylenedi-phosphonic acids (H0)2P(0)CH(NR R )2P(0)(0H)2 has been prepared from phosphorus trihalides and NN-dialkylformamides. Acid dissociation constants and the complex behaviour toward Cu " for the phosphinyl-propionic acid Ph(H0)P(0)CMe(0H)(C02H) have been evaluated. ... 

As in Section 14.3.3, we shall deal only with the rapid equilibrium of substrate and hydrogen-ion binding, because the full steady-state equations are very complex. Hence, the Michaelis constant represents the true dissociation constant of respective enzyme-substrate complexes, and Kp and Kb represent the acid dissociation constants of amino acids in the free and the bound enzyme, respectively. 

Rossotti et al [Fa 70] found that the ratio of stability constants obtained by equilibrium measurements in solvent mixtures similar to the above was independent of the composition of the solvent. McBryde et al showed that in most of the systems they examined, the ratio of the first two stepwise complex stability constants did not depend on the solvent. In the case of the stepwise acid dissociation constants this correlation was approximately true only for ethylenediamine for glycine the ratio of the protonation constants increase with increase in the concentration of the organic component. However, if the formation of the glycine complexes is defined as a reaction between the protonated ligand and the metal ion, accompanied by the liberation of a hydrogen ion, the ratio of the equilibrium constants is approximately independent of the composition of the solvent mixture. 

---