Formation constants halides

At(0) reacts with halogens X2 to produce interhalogen species AtX, which can be extracted into CCI4, whereas halide ions X yield polyhalide ions AtX2 which are not extracted by CCLt but can be extracted into Pr O. The equilibrium formation constants of the various trihalide ions are intercompared in Table 17.28. 

We begin by considering the stability constants for the formation of halide complexes with zinc(ii) and mercury(ii) (Table 9-2)... 

The reaction between Fe(IlI) and Sn(Il) in dilute perchloric acid in the presence of chloride ions is first-order in Fe(lll) concentration . The order is maintained when bromide or iodide is present. The kinetic data seem to point to a fourth-order dependence on chloride ion. A minimum of three Cl ions in the activated complex seems necessary for the reaction to proceed at a measurable rate. Bromide and iodide show third-order dependences. The reaction is retarded by Sn(II) (first-order dependence) due to removal of halide ions from solution by complex formation. Estimates are given for the formation constants of the monochloro and monobromo Sn(II) complexes. In terms of catalytic power 1 > Br > Cl and this is also the order of decreasing ease of oxidation of the halide ion by Fe(IlI). However, the state of complexing of Sn(ll)and Fe(III)is given by Cl > Br > I". Apparently, electrostatic effects are not effective in deciding the rate. For the case of chloride ions, the chief activated complex is likely to have the composition (FeSnC ). The kinetic data cannot resolve the way in which the Cl ions are distributed between Fe(IlI) and Sn(ll). 

Fig. 8. Correlation between Pearson s hardness parameter (7P) derived from gas-phase enthalpies of formation of halide compounds of Lewis acids (19), and the hardness parameter in aqueous solution (/A), derived from formation constants of fluoride and hydroxide complexes in aqueous solution (17). The Lewis acids are segregated by charge into separate correlations for monopositive ( ), dipositive (O), and tripositive ( ) cations, with a single tetrapositive ion (Zr4+, ). The /P value for Tl3+ was not reported, but the point is included in parentheses to show the relative ionicity of Tl(III) to ligand bonds. 

The effect of halide, cyanate, cyanide, and thiocyanate ions on the partitioning of Hg in [BMIM][PF6]/aqueous systems (Figure 3.3-2) has been studied [8]. The results indicate that the metal ion transfer to the IL phase depends on the relative hydrophobicity of the metal complex. Hg-I complexes have the highest formation constants, decreasing to those of Hg-F [42]. Results from pseudohalides, however, suggest a more complex partitioning mechanism, since Hg-CN complexes have even higher formation constants [42], but display the lowest distribution ratios. 

Mixed halide-thiocyanate compounds Hg(SCN)X (X = Cl, Br, I) are formed from equimolar amounts of the pure components. They contain six-coordinated mercury(H) achieved by bridging X and SCN groups.234 The formation constants of the mixed thiocyanato complexes have been detected spectroscopically.233 Raman spectra of mixed halothiocyanatomercurate(U) complexes have been reported by Cooney and Hall.236 The structure of ammonium... 

These very small values for the thermodynamic functions tend to suggest that chloride ions form very weak complexes indeed if not just forming an ion-pair. The first formation constant, ki, shows a decrease with increase in ionic radii of the halide ions (Table 17). 

The ability of Hg(II) to form tri- and tetra-coordinated ions with suitable ligands is well known. With alkylmercuric halides the formation constants are usually too small for such ions to be conventionally demonstrated (Brown et al., 1965a). It is likely, however, that such complexes would cleave more readily than the uncomplexed materials. These expectations have been strongly supported in a study of the cleavage of allylmercuric iodide by acid and iodide ion (Kreevoy et al., 1966a equations (6) to (8)). The rate was of the form shown in equation (23), in which S is the substrate. The terms which are linear in iodide ion... 

Table 10. Solubility products of silver halides, and formation constants of dihalogenido complexes of the silver ion in various solvents...

The possibility of e+ bound-state formation has attracted the attention of theoreticians [51-55], as exposed elsewhere in this book. However, discrepancies may appear when comparing (kinetic) data from AC and DB and (thermodynamic) theoretical predictions. For the halides for instance, theory predicts an order of stability for the positron bound-states as F" > CF > Br > r, while the experimental bound-state formation constants are in the reverse order. An evident reason for this arises from the solvation energies of the ions, not included in the theoretical evaluations, which decrease importantly the trap depth of F towards e+. 

Halide Ligands. The formation constants in aqueous solution are very low, for example,... 

In halide solutions the equilibrium concentrations of the various possible species depend on the conditions although CuClf- has only a low formation constant, it is precipitated from solutions by large cations of similar charge (see above). 

Halogeno Complexes. The halides AgX react with halide anions X" to give a series of anionic complexes [AgkX]](1 lt), with relative stabilities L > Br > Cl . The formation constants of [AgX2] ions vary considerably with the solvent and are quite low in water. Nonetheless, AgCl is about 100 times more soluble in 1 M HC1 than in pure water. 

Maciel et al. (243) have determined the shifts for the halide complexes ZnX " (n = 0,1,2,3,4 X = Cl , Br ) on the basis of the observed dependence of Zn " shieldings on halide concentration and individual formation constants available from other sources. For this purpose they developed a least-squares treatment which has permitted them to fit the experimental shieldings to ... 

There is no marked change in the formation constants of the halides as X is varied, i.e., there is no change from strongly class (a) to class(6) character 11) or vice versa. 

Table VIII, part B), to compare directly the cis and trans effects of one ligand X (Table VTII, part C), and to see the effect of successive replacement of H2O by NH3 (Table VIII, part D). Formation constants involving the halides are given in Table VIII, part E, and some constants involving other ligands in Table VIII, part F. 

A practical problem frequently encountered by the analytical chemist is that of maintaining as nearly as possible a constant ionic environment while varying the concentration of one component of a mixture of electrolytes. For example, in evaluating the successive formation constants of halide complexes of a metal ion, it is necessary to vary the halide ion concentration over wide limits. The practice generally... 

Most interesting are the selectivity differences between an alumina phase and a strongly basic anion exchanger with quaternary ammonium groups. In comparison with a conventional anion exchanger, halide ions, for example, are eluted in reverse order I-< Br < Cl < F. This order corresponds to the formation constants of aluminum halide complexes [46], which suggests an interaction between aluminum and halide ions within the A1203 structure. 

We see that only complexes with formation constants of the order of 106 M-1 or more will lead to titration curves with a sufficiently steep change in pL near the equivalence point (at CM VM / CL VL = 1) to be useful for volumetric analysis. None of the common monodentate ligands, such as the halide anions (Cl-, Br , I-) or the pseudohalides (CN , SCN-, N3 ), form such strong complexes, nor do the carboxylic acid anions (such as acetate) or ammonia (NH3). However, in section 5.2 we will encounter special ligands, the chelates, that do form sufficiently strong 1 1 complexes. 

Complex anions with halides are formed by both metals but the formation constants differ widely they are also many orders of magnitude smaller than those of Hg2 +, as is clear from Tables 18-7 and 18-8. The exact values are... 

Some Formation Constants of Zinc and Cadmium Halide Complexes (at 25°)... 

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