Stability constants of coordination complexes

Solvent extraction involves the extraction of a substance using a suitable solvent in a two-phase solvent system, the solute is extracted from one solvent into another, the extracting solvent being chosen so that impurities remain in the original solvent. 

As we saw earlier, metal ions in aqueous solution are hydrated. The aqua species may be denoted as M (aq) where this often represents the hexaaqua ion [M(OH2)6l. Now consider the addition of a neutral ligand L to the solution, and the formation of a series of complexes [M(OH2)5Lf+, [M(0H2)4L2] +,. .., [ML6f+. Equilibria 7.65-7.70 show the stepwise displacements of coordinated H2O by L. 

Nuclear fission can be successfully harnessed to produce nuclear energy. An advantage of nuclear energy is that it is not associated with emissions into the atmosphere of CO2, SO2 and NO c (see Boxes 14.9, 16.5 and 15.7). Disadvantages include the problems of disposing of radioactive isotopes generated as fission products, and the risks involved if a nuclear reactor goes critical . 

Catastrophic branching chain reactions are prevented by inserting boron-containing steel or boron carbide rods which control the number of neutrons. 

Eventually, the 92 U fuel is spent, and requires reprocessing. This recovers uranium and also separates from the fission products. First, the spent fuel is kept in pond storage to allow short-lived radioactive products to decay. The uranium is then converted into the soluble salt [U02][N03]2 and finally into UFs  

The equilibrium constant,, for reaction 6.65 is given by equation 6.71 [H2O] (strictly, the activity of H2O) is unity 

In Section 2.5, we discussed the production of energy by nuclear fission, and the reprocessing of nuclear fuels. We described how short-lived radioactive products decay during pond storage, and how uranium is converted into [U02][N03]2 and, finally, UFg. One of the complicating factors in this process is that the fuel to be reprocessed contains plutonium and fission products in addition to uranium. Two dilferent solvent extraction processes are needed to elfect separation. 

Stage 1 separation of the fission products from plutonium and uranium nitrates 

The mixture to be separated contains [U02] and Pu(TV) nitrates, as well as metal ions such as 3gSr. Kerosene is added to the aqueous solution of metal salts, giving a two-phase system (i.e. these solvents are immiscible). Tributyl phosphate (TBP, a phosphate ester) is added to form complexes with the uranium-containing and plutonium ions, extracting them into the kerosene layer. The fission products remain in the aqueous solution, and separation of the solvent layers thus achieves separation of the fission products from Pu- and U-containing species. Repeated extractions from the aqueous layer by the same process increases the efficiency of the separation. 

In the formation of a complex [MLg] from [M(OH2)6f, each displacement of a coordinated water molecule by ligand L has a characteristic stepwise stability constant, K, K2, K, 4, or K.  

Alternatively, we may consider the overall formation of [MLg] (equation 7.72). In order to distinguish stepwise and overall stabihty constants, the symbol /3 is generally used for the latter. Equation 7.73 gives an expression for j3f, for [MLg]. We must refer to /3g and not just /3, because overall stability constants for the products of each of reactions 7.65-7.70 can also be defined (see problem 7.25 at the end of the chapter). 

Values of K and / are related. For equilibrium 7.72, / g can be expressed in terms of the six stepwise stability constants according to equations 7.74. 

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The stability constants of zinc complexes of 4,6-dimethyl-2-thiopyrimidine have been determined by potentiometric measurements. The crystal structure shows infinite zigzag chains of ZnL2 units with each zinc coordinated by an N3S2 donor set in a trigonal-bipyramidal geometry.853... 

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+. 

We can now make sensible guesses as to the order of rate constant for water replacement from coordination complexes of the metals tabulated. (With the formation of fused rings these relationships may no longer apply. Consider, for example, the slow reactions of metal ions with porphyrine derivatives (20) or with tetrasulfonated phthalocyanine, where the rate determining step in the incorporation of metal ion is the dissociation of the pyrrole N-H bond (164).) The reason for many earlier (mostly qualitative) observations on the behavior of complex ions can now be understood. The relative reaction rates of cations with the anion of thenoyltrifluoroacetone (113) and metal-aqua water exchange data from NMR studies (69) are much as expected. The rapid exchange of CN " with Hg(CN)4 2 or Zn(CN)4-2 or the very slow Hg(CN)+, Hg+2 isotopic exchange can be understood, when the dissociative rate constants are estimated. Reactions of the type M+a + L b = ML+(a "b) can be justifiably assumed rapid in the proposed mechanisms for the redox reactions of iron(III) with iodide (47) or thiosulfate (93) ions or when copper(II) reacts with cyanide ions (9). Finally relations between kinetic and thermodynamic parameters are shown by a variety of complex ions since the dissociation rate constant dominates the thermodynamic stability constant of the complex (127). A recently observed linear relation between the rate constant for dissociation of nickel complexes with a variety of pyridine bases and the acidity constant of the base arises from the constancy of the formation rate constant for these complexes (87). 

Dale Margerum Ralph Wilkins has mentioned the interesting effect of terpyridine on the subsequent substitution reaction of the nickel complex. I would like to discuss this point—namely the effect of coordination of other ligands on the rate of substitution of the remaining coordinated water. However, before proceeding we should first focus attention on the main point of this paper-which is that a tremendous amount of kinetic data for the rate of formation of all kinds of metal complexes can be correlated with the rate of water substitution of the simple aquo metal ion. This also means that dissociation rate constants of metal complexes can be predicted from the stability constants of the complexes and the rate constant of water exchange. The data from the paper are so convincing that we can proceed to other points of discussion. 

Clearly the position of the homogeneous reaction (la) will depend on the concentration of the free metal ions which can be modified by an auxiliary complexing (or masking) agent (see Section 10.3). It will move increasingly to the right as the (overall) stability constant of the complex, ML , increases and to the left as the solution becomes more acidic. Increase of pH should lead to more complete reaction but since this implies a concomitant increase in hydroxyl ion concentration there will now be increasing competition between the tendencies of IT and OH- to coordinate to the cation basic species and even metal hydroxides may form and precipitate. 

Other Coordination Complexes. Because carbonate and bicarbonate are commonly found under environmental conditions in water, and because carbonate complexes Pu readily in most oxidation states, Pu carbonato complexes have been studied extensively. The reduction potentials vs the standard hydrogen electrode of Pu(VI)/(V) shifts from 0.916 to 0.33 V and the Pu(IV)/(III) potential shifts from 1.48 to -0.50 V in 1 M carbonate. These shifts indicate strong carbonate complexation. Electrochemistry, reaction kinetics, and spectroscopy of plutonium carbonates in solution have been reviewed (113). The solubility of Pu(IV) in aqueous carbonate solutions has been measured, and the stability constants of hydroxycarbonato complexes have been calculated (Fig. 6b) (90). 

The coordinating ability (i.e. the stability constants of the complexes) of sulfur-containing ligands increases with the dipole moment in the order H2S < RSH < R2S. The stability of the complexes decreases in the order S2 > RS > R2S. Here the polarizability and the number of lone pairs are the dominant factors. 

The complexing ability of the PVI has been studied with several heavy metal ions 89,90) stability constants of the complexes have been determined in aqueous solution by the method of Bjerrum91), modified for the binding of metals by polyelectrolytes 92>. It could be concluded that both Cu2+ and Zn2+ ions coordinate four imidazolyl groups. (Table 6). 

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