Stability constants halide, bromide complexes

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. 

Proton nmr halide anion titrations reveal that the ethyl- [79], propyl-[80] and butyl- [81] linked derivatives (Fig. 43) form complexes of 1 1 stoichiometry in acetonitrile solution. Stability constant determinations suggest that the ethyl derivative [79] exhibits selectivity for the chloride anion in preference to bromide or iodide. As the chain length increases, so the selectivity for chloride decreases and also the magnitude of the stability constant which is evidence for an anionic chelate effect with the chloride anion. Receptors containing larger aryl [81], [83], [84] and alkylamino spacers [85] (Fig. 43) form complexes of 2 1 halide anion receptor stoichiometry. 

The stability constants in melts of NH4N03- H20 of ZnX+, ZnX2 (n = 1-3 X = Cl or Br), CdX+, CdX2 (n = 1.5-3 X = Cl or Br) and HgX HgX2 (n = 2.5 X = Cl or Br) have been determined.950,931 The behaviour of zinc is peculiar if the Ki and K2 values are compared with those of cadmium and mercury. The stability constants increase with temperature and the bromide is more stable than the chloride, trends which are opposite to those normally observed for the halide complexes of most metals in anhydrous or aqueous melts. The data also show... 

Table 11.2 gives some stability constants for complexes of Th + and U02 +. Thorium forms stronger complexes with fluoride, the hardest halide ion, than with chloride and bromide this is the behaviour expected of a hard Lewis acid. Thorium also forms quite... 

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... 

As is to be expected from the soft character of Tl , strong chloride and bromide complexes of type T1X " are formed in solution, where n is less than or equal to 4. These species are more stable in aqueous solution than the analogous Al Ga and In halides. Some of these halides have been thoroughly characterized both in the solid state and in solution by means of X-ray crystallography, solution X-ray diffraction, and NMR. These complexes are among the most stable metal-halide complexes known, particular stability having been claimed for the TIX2+ complexes with chloride (see e.g. the overall stability constants, = 10 10 ° 10 10 8.32 q18.02. respectively for X = Cl in... 

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). 

These data reveal some important aspects of the inclusion phenomenon displayed by the anion hosts 24 and 25 Many small and heavily hydrated anions can be bound in water with stability constants exceeding those with simple cyclodextrinsby factors of 20-150. The specificity of binding, however, is rather poor. The stability pattern does not follow the hydration enthalpies of the anions as is most obviously realized in the halide series. Though bromide ion possesses an intermediate enthalpy of hydration it forms the most stable inclusion complex with 24. 

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