Metal ions, hydrolysis constants Table

There is no reason to believe that the conjugate base mechanism does not apply with the other metal ions studied. Complexes of Cr(III) undergo base hydrolysis, but generally rate constants are lower, often 10 —10 less than for the Co(III) analog, Table 4.10. The lower reactivity appears due to both lower acidity (A"i) and lower lability of the amido species (kf) in (4.49) (provided k i can be assumed to be relatively constant). The very unreactive Rh(III) complexes are as a result of the very low reactivity of the amido species. The complexes of Ru(III) most resemble those of Co(III) but, as with Rh(III), base hydrolyses invariably takes place with complete retention of configuration. ... 

Many formation constants involve polycarboxylates Table 28 summarizes the data. Nagyp l and Fabian s report on the oxalic and malonic systems seems the most complete as hydrolysis of both metal ion and complexes has been included.584 A concentration distribution of the complexes in the malonic system is shown in Figure 25. The order of basicities is succinic > citraconic > itaconic > maleic > malonic acid and log /3U0 should follow the same order. However, from Table 28, the order of stabilities is citraconic > malonic > maleic > itaconic > succinic acid.608... 

The definition of pH is pH = —log[H+] (which will be modified to include activity later). Ka is the equilibrium constant for the dissociation of an acid HA + H20 H30+ + A-. Kb is the base hydrolysis constant for the reaction B + H20 BH+ + OH. When either Ka or Kb is large, the acid or base is said to be strong otherwise, the acid or base is weak. Common strong acids and bases are listed in Table 6-2, which you should memorize. The most common weak acids are carboxylic acids (RC02H), and the most common weak bases are amines (R3N ). Carboxylate anions (RC02) are weak bases, and ammonium ions (R3NH+) are weak acids. Metal cations also are weak acids. For a conjugate acid-base pair in water, Ka- Kb = Kw. For polyprotic acids, we denote the successive acid dissociation constants as Kal, K, K, , or just Aj, K2, A"3, . For polybasic species, we denote successive hydrolysis constants Kbi, Kb2, A"h3, . For a diprotic system, the relations between successive acid and base equilibrium constants are Afa Kb2 — Kw and K.a Kbl = A w. For a triprotic system the relations are A al KM = ATW, K.d2 Kb2 = ATW, and Ka2 Kb, = Kw. 

Initially studies of metal ion-promoted hydrolysis were centred on simple monoamin esters.36,44,43 However, many of the initial investigations led to rather conflicting results. Th reactions are difficult to study due to the low formation constants of the active complexes. Mor recent measurements46 48 have provided rate constants (Table 4) which show only order c magnitude agreement however, it has been possible to establish that hydroxide ion is th predominant nucleophile at pH values of ca. 5. Higher pH values lead to precipitation of meti hydroxides. Evidence for nucleophilic attack by water has also been obtained.46"48... 

Table 4 Rate Constants for the Metal Ion-promoted Hydrolysis of Methyl Glycinate and Ethyl Glycinate...

Table 9 Rate Constants fcOH f°r the Metal Ion-promoted Base Hydrolysis of some Bidentate Amino Acid Ester Species...

Metal ions have quite marked effects on the hydrolysis of methyl 8-hydroxyquinoline-2-carboxylafe (50)218 and ethyl l,10-phenanthroline-2-carboxylate (51).219 Base hydrolysis of the 8-hydroxyquinoline derivative (50) was studied over the pH range 9.2-12.1 and values of fcoH determined for HA and the anion A-. Formation constants KMA+ were determined at 25 °C for the equilibrium M2+ + A MA+, as were the rate constants fcOH for the base hydrolysis of the MA+ complexes (Table 19). Quite large rate accelerations are observed (103-106) when comparisons are made with the base hydrolysis of A . The charge carried by the complex does not appear to be a major factor in determining base hydrolysis rates. Thus at 25 °C the complex CuA+ undergoes base hydrolysis (kOH= 6.3xl05 s 1) at a very similar rate to the corresponding... 

To understand the role of metal ions in hydrolysis reactions, it is useful to first consider the background hydrolysis reactions. Table 6.1 lists the second-order rate constants for hydroxide-catalyzed hydrolysis of various substrates. The reactivity of methyl acetate (first entry in Table 6.1) [16] is comparable to those of other unactivated esters found in nature (e.g. acetyl choline and carboxyl esters in phospholipids). The reactivity of N-methylacetamide (second entry in Table 6.1) [17] is comparable to those of typical peptides (1.1 x 10 6 m-1 s 1) [18] and that of dimethyl phosphate (P-O bond... 

Table 2A. Solubility Products and Hydrolysis Constants of Metal Ions"...

Chapters 3-5 have described the calculation of various transformed thermodynamic properties of biochemical reactants and reactions from standard thermodynamic properties of species, but they have not discussed how these species properties were determined. Of course, some species properties came directly out of the National Bureau of Standard Tables (1) and CODATA Tables (2). One way to calculate standard thermodynamic properties of species not in the tables of chemical thermodynamic properties is to express the apparent equilibrium constant K in terms of the equilibrium constant K of a reference chemical reaction, that is a reference reaction written in terms of species, and binding polynomials of reactants, as described in Chapter 2. In order to do this the piiTs of the reactants in the pH range of interest must be known, and if metal ions are bound, the dissociation constants of the metal ion complexes must also be known. For the hydrolysis of adenosine triphosphate to adenosine diphosphate, the apparent equilibrium constant is given by... 

The similarities between Ga, In and Fe " are manifest in vivo by the binding of all three ions to the serum protein transferrin, Tf, normally used for iron transpQit. The formation constant for the Ga -Tf complex has been found to be and Welch has calculated values for the equilibrium constants for the exchange of trivalent metal ions between EDTA or DTPA and Tf as shown in Table 19. These figures show that only the DTPA complex of Ga is stable with respect to metal exchange with Tf. Table 19 also shows values for the equilibrium exchange reaction between Tf and hydroxide ion. These indicate that, while the indium-Tf complex should be stable to hydrolysis in vivo, in the long term the insoluble Ga(OH)3 should form from the... 

Values of p/fh from Yatsimirksii. K. B. Vasil ev, V. P. instability Constants of Complex Compounds Pergamon Elmsford, NY. 1960, except for Bi, Hf, Lu. Pu, Sc, and Tl. which arc from Stability Constants of Metal-Ion Complexes Part II, Inorganic Ligands Bjenum, J. Schwarzenbach. 0. Sillen, L. G.. Eds. The Chemical Society London, 1958. For many elements there is considerable uncertainty in the hydrolysis constants not only as a result of experimental errors but also because some have not been corrected to infinite dilution. Z /r values were calculated from ionic radii in Table 4.4. 

TABLE 1.5 Acid Hydrolysis Constants (pAas) for Some Aquo Ions and Complex Metal Ions at 25 °C (Selected from Basolo and Pearson,22 and pH Precipitation Ranges of Hydroxides or Hydrous Oxides (Adapted from Walton23)... 

The rate enhancements brought about by various metal ions for the hydrolysis of penicillins and cephalosporins are summarised in Tables 7 and 8. There is no correlation between the binding constant of the p-lactam antibiotic with the metal ion and the rate enhancement. The order of reactivity is that of the Irving-Williams series (Irving and Williams, 1953) Co(II) < Ni(II) < Cu(II) > Zn(II). 

---