Complexes with halide ions

The EX4 halides are Lewis acids that form complexes with halide ions,... 

The basic salts of quadrivalent polonium, such as the sulfate and sele-nate, show a marked resemblance to those of tellurium and further resemblances appear in the quadrivalent halides, particularly in their complexing with halide ions in solution, while complexing of polonium(IV) with weak acids, such as acetic, oxalic and tartaric, seems to be more marked than is the case with tellurium. 

The solvation complexes, Fe(H20)g+ and Hg(H20>4+, can form complexes with halide ions according to the scheme in Eq. 2.5a. Some thermodynamic data for the overall complexation reactions at 298 K follow. Use these data to calculate ArH° and then explain the differences among the values for the metal-ligand adducts. (Hint See Eq. 2.4.)... 

It also forms complexes with halide ions in both aqueous and non-aqueous media the ion S02F" (page 456) can be regarded as an especially stable solvate.53... 

Ferric ion forms complexes with halide ions and SCN. Its affinity for F is quite high, as shown by the equilibrium constants... 

HSAB principle A method of classifying Lewis acids and bases (see acid) developed by Ralph Pearson in the 1960s. The acronym stands for hard and soft acids and bases . It is based in empirical measurements of stability of compounds with certain ligands. Hard acids tend to complex with halide ions in the order F->Cl->Br>I-... 

Table 2 Reactions of metal complexes with halide ions and halogen radicals ...

While nickel(III) cyclam, [NiL(OH2)2], is known to form complexes with halide ions (Cl", Br"), the redox potential (0.96 V) is sufficiently high that iodide is oxidized quantitatively.The rate law, however, shows only a first-order dependence in the iodide concentration. Although not conclusive, the preferred mechanism for reaction is via an outer-sphere process when rates are compared with those for halide complex formation. 

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. 

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