Cobalt complexes stability constants

Amine-extraction equilibria can also be modeled by chemical-reaction equilibrium constants. Figure 8.3-3 indicates that cations such as iron(III), zinc, cobalt(lI) and copper(II) exhibit high distribution coefficients with chloride solutions, whereas nickel, iron(II), and manganese are not extracted to any great extent. The basis for the differences in distribution coefficients lies mainly in the tendency for the former group of cations to form chloride complexes. Stability constants for these complexes are available in the literature," and they can be used to develop quantitative phase-equilibrium models. 

For TIOA with hydrochloric acid the concentration-based equilibrium constant for salt formation" according to reaction (8.2-6) is 1.51 x 10 and the equilibrium constant for amine-hydrochloride salt dimerization" is 8.0 M Combination of these parameters and the ion-complex stability constants with experimental metal-distribution data allows determination of the equilibrium constants for reactions (8.2-5) or (8.2-7). This completes the description of the amine-metal extraction-phase equilibria. For cobalt(II) in acidic sodium chloride solutions the equilibrium constant" for reaction (8.2-7) with TIOA is 2.0 X 10 and that for coppeifll) is 370 The corresponding value for zinc" is 7.5 x 10 Af -In spile of these relative values, the order of selectivity of TIOA for extraction of the metals is Zn > Cu > Co because of the relative extent of chloride complex formation. For the same reason, zinc stripping is difficult in this system, and copper has a tendency to be reduced to cuprous, which also complexes and extracts extensively. 

Mapsi et al. [16] reported the use of a potentiometric method for the determination of the stability constants of miconazole complexes with iron(II), iron(III), cobalt(II), nickel(II), copper(II), and zinc(II) ions. The interaction of miconazole with the ions was determined potentiometrically in methanol-water (90 10) at an ionic force of 0.16 and at 20 °C. The coordination number of iron, cobalt, and nickel was 6 copper and zinc show a coordination number of 4. The values of the respected log jSn of these complexes were calculated by an improved Scatchard (1949) method and they are in agreement with the Irving-Williams (1953) series of Fe2+ < Co2+ < Ni2 < Cu2+ < Zn2+. 

Willems et al. [37] used a polarographic method to study the miconazole complexes of some trace elements. Manganese, iron, cobalt, and zinc element formed miconazole complexes with different stability constants. Polarography was used for detecting stability constants. The evolution of the respective formation constants followed the natural (Irving-Williams) order. The stepwise constant of the complexes formed increased from manganese to cobalt and decreased for zinc. The results are discussed with respect to the possible mechanism of action of miconazole. 

Ethylenediaminetetraacetic acid, analogs, complexes of, 3 277 chelation by, 3 276-277 cobalt complex of, 3 281 complexes, 3 277-278 formation constant of, 3 273-274 -nickel, 3 17-18 stability of, 3 266-267 reaction with metal ions, 3 62 Ethylene dibromide, irradiation of, 5 196 4,5-Ethylenedithio-1,3-dithiole-2-thione based supramolecular complexes, 46 200-204 Ethylene glycol, 32 4... 

Measurements of the equilibrium constants of the reactions imply that the stabilities of the monosubstituted complexes are predominantly determined by steric effects of the ligand, reflecting a very crowded space around the metal in this system. The large ligands PPh3 or P(c-C6H,) do not react with the cobalt complex. 

There has been some uncertainty concerning the metal content of alkaline phosphatase and the role of zinc in the catalytic process. Early measurements by Plocke et al. (36, 50) showed that there were 2 g-atoms per dimer. The zinc requirement for enzymic activity was demonstrated by the inhibition of the enzyme with metal binding agents in accord with the order of the stability constants of their zinc complexes. It appears that in some cases (EDTA) zinc is removed from the enzyme and in other cases (CN) the ligand adds to the metalloprotein. A zinc-free inactive apoenzyme was formed by dialysis against 1,10-phenanthro-line. Complete activity was restored by zinc only zinc, cobalt, and possibly mercury produce active enzyme. 

Another example of an organometallic complex that has been found in natural waters is vitamin B12. The stability constant of the cobalt in B12 must be large since both cobalt and Bi2 exist in natural waters at less than 1 / gram/liter. 

For catalytic waves of hydrogen evolution in ammoniacal cobalt solutions, it has been observed (132) that ery/Aro-phenylcysteine gives a higher catalytic wave than the threo form (Fig. 28). These differences can be explained partly by differences in acid dissociation constants, and partly by variations in the stability constants of the cobalt-phenyl-cysteine complexes. 

The first-row transition metal ions have borderline hard soft properties. Therefore, there is no simple rule to predict comparative complex stabilities for a given metal ion with the different donor atoms. In contrast, the stability of complexes of these metal ions with a given ligand does follow a general trend. Thus the stability constants for complexes with NTP increase with the metal ion in the order Co + < Mn + < Zn + < Ni " " < Cu " ", which is the usual Irving-Williams order, except for the reversal of cobalt and manganese. The usual decrease in stability with increasing atomic number is observed for the complexes with metal ions of the alkaline earth series. The stabihty of the... 

Since ammonia forms stable, water-soluble complexes with many metals, leaching can be carried out under alkaline conditions to give these metals in solution. Of particular interest are the metals copper, nickel and cobalt, which form particularly stable amines lliat have been well characterized as having the following approximate stability constants (at high ionic strength) Cu, 2 = Cu , 4 = 13 Ni , 6 = 9 = 5 Fe ,j52 < 2. 

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