Stability constants, metal-iodide complexes

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). 

Metal-iodide complexes are not likely to be very important in seawater because there is a higher concentration of chloride (6 orders of magnitude higher) and bromide (3 orders of magnitude). Although the stability constants for metal-iodide complexes are stronger than for chloride and bromide com-... 

There is a significant difference in the affinity of the two metal irais towards the complexation with halide ions, too. In a diluted aqueous solution of zinc(II) chloride the octahedral aqua ions [Zn(H20)e] are the predominating species, while a significant ratio of chloro complexes are formed with cadmium(II) under the same conditions. The stability constants of the complexes formed with bromide or iodide ions are even higher. The consideration of these differences is especially important during the selection of the appropriate counter ions to adjust the ionic strength for thermodynamic or electrochemical studies or even in the synthesis of cadmium(II) compounds. 

In contrast to the potassium and caesium halide melts, a monotonous change of the dissociation constant of carbonate ion in the sequence of sodium halides was observed. This distinction has been explained on p. 147 by the different stabilities of the inner complexes formed by melt ions in individual molten alkali metal halides, and by the different character of their changes with the change in the melt anion. The stability of the complex changes greatly in the potassium and caesium chloride-bromide-iodide sequences (the minimum is observed in the bromide melts), whereas in the sodium halides the chloride complexes possess the lowest stability and the iodide... 

NMR spectroscopy revealed that upon the addition of 1 equiv. of alkali metal halide salts to receptors 78a-c, 1 1 complexes were formed. Stability constants were measured using halide electrodes in water solutions, which revealed that the receptors bound bromide anions (logKa = 1.8, 2.45 and 2.45 for 78a, 78b and 78c, respectively) and iodide anions (logKa = 2.2 and 2.4 for 78b and 78c, respectively). 

High-precision and high-accuracy determination of amino acids is difficult. Let us examine the possibilities of an indirect approach through dissolving a metal ion compound followed by the highly accurate EDTA (or other) metal ion methods. An iodide-Cu(II) method was proposed in 1950. Likely candidates are Cu(II) and Hg(II), which form very stable complexes with amino acids. The glycinate complexes have the stability constants shown in Table 11-3. The objective is to get quantitative dissolving of a low solubility metal ion compound when excess of it is stirred with a measured... 

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