Halide and pseudohalide ions

Complex Atj (l.mole. sec ) Temperature E (°C) kcal.mole ) ASHeu) 

The authors acknowledge that the present data are insufficient to allow a detailed mechanism to be given but they reject the possibility of slow 8 2 substitution followed by a rapid redox process. 

The oxidation of potassium thiocyanate by AuBrJ is first-order in oxidant and the pH dependence indicates that it is also first-order in thiocyanate ion, which is oxidised much faster than HCNS . The activation parameters are E = 6.4+0.4 kcal.mole and A5 = 26 + 2 eu. 

Bi(V) in aqueous perchloric acid is very strongly oxidising but kinetic studies have been confined to a few stopped-flow measurements on oxidation of iodide, bromide and chloride ions. The appearance of Bi(III)-halide complexes was first-order with respect to Bi(III) and in all cases the first-order rate coefficient,, was the same, i.e. 161 + 8 sec at 25 °C ([H30 ] = 0.5 M, p. = 2.0 A/), irrespective of the nature or concentration of the halide. A preliminary attack on solvent is compatible with these interesting results, viz. 

These authors also cite unpublished work on Sb(V) oxidation of I which follows a rate law involving the term [Sb(V)][I ].  

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Reactions of a variety of weaker nucleophiles such as halide and pseudohalide ions and [Pt(PPh3)2(CNCH3)2] have been studied (747) and, as might be expected, ligand replacement is usually observed [Eq. (11)]. 

In Table 6 the differences of free enthalpies of solvation for several anion ligands in a donor solvent D and in AN are given. HMPA shows very weak solvation whereas water is a very strong solvating agent for anions. The free enthalpies of solvation of halide and pseudohalide ions are by 4 to 15 keal/mol more negative than in aprotic donor solvents. 

Molecules in the second coordination sphere, that is, solvent or partners of an ion pair, can transfer charge to the metal ion of a coordination complex in an optically induced electronic transition. The excited states are, respectively, known as solvent to metal charge transfer (SMCT) and ion pair charge transfer (IPCT) excited states. A typical example are the ion pairs, [Cora(NH3)6]3 +, X-, formed between [Coln(NH3)6]3 + and halide and pseudohalide ions X. The UV-Vis spectrum of ion... 

Prosthetic heme modification also occurs in some instances with the two-electron oxidation products formed by peroxidases when they oxidize halide and pseudohalide ions. Thus, the oxidation of bromide by HRP results in the addition of HOBr to one or both of the heme vinyl groups (Fig. 5.11) [62], Similar reactions are... 

Direct, two-electron oxidations are rare for most peroxidase enzymes. The one broad exception is the oxidation of halide and pseudohalide ions, specifically F, Br , Cl-, and NCS . Fluoride ion, in contrast, is not known to be oxidized by these enzymes. The oxidation of F and NCS" is common for the peroxidases, whereas that of Br is widespread but is usually less effective, and that of Cl , among the conventional peroxidases, is only important in the case of MPO [46, 83]. The halogenation activities of the mammalian peroxidases are compared in Table 5.3. As the table shows, chloride ion is oxidized by MPO, particularly at pH 5, but it is a very poor substrate for EPO and LPO. Br , F, and SCN" are readily oxidized by all three enzymes, but most efficiently by EPO at pH 5 [84—86]. 

The reducing power of the halide and pseudohalide ions increases in the following order F , NCO , Cl , N3, Br , SCN , I . In this and other respects the azido group shows close resemblance to bromine (Table 1). 

Complex Formation of Co + with Halide and Pseudohalide Ions. 126... 

There are therefore two main reasons for the laige deviations observed for water (a) entropic effects and enthalpic effects by hydrogen bonding interactions between water molecules of neighbouring hydration shells, as discussed in the preceding section, and (b) the strong solvating power of water towards halide and pseudohalide ions. 

Table I. Classification of Halide and Pseudohalide Ions in [Al2(CH3)6X] as to the Formation of LiquidClathrates with Benzene...

Minisci, F. and Fontana, F. (1994) Mechanism of the Gif-Barton Type Alkane Functionalization by Halide and Pseudohalide Ions, Tetrahedron Lett. 35, 1427-1430. 

Large numbers of compounds have been prepared that contain the rare earth ion coordinated to the halide and pseudohalide ions and representative examples are given in table 25.11. 

Complexes formed by heterocyclic amines with halides and pseudohalides of dipositive first-transition-series metal ions are usually prepared by the reaction between a metallic salt, a soluble thiocyanate, and pyridine in aqueous solution. Although the number of coordinated pyridine molecules varies,... 

Addition of excess iodide to the insoluble Hgl2 results in the formation of soluble mercury iodo complex [Hgl3] , with a trigonal planar structure. The ion is solvated in water and converts to a tetrahedral structure. Further, addition of H leads to tetrahedral [Hg ] ". Reaction of iodide salts with Hg can be used to produce mercury iodo complexes. Other halide and pseudohalides also form [HgXj] and [HgX4] . The tetrahalo anions see Anion) are usually tetrahedral, while the trihalo ions readily add solvent molecules to form distorted tetrahedral or Trigonal Bipyramidal structures. 

In this reaction an anionic leaving group is replaced by a neutral donor molecule. Consequently, if relative EPD strengths of both neutral and anionic EPD units are known, it should be possible to make at least qualitative predictions about the ionizability of substrates A—Bj, A—Bg,. .. in various EPD solvents. Approximate values of relative EPD strength of halide, pseudohalide, and neutral EPD units have been determined using vanadyl(IV)acetylacetonate, VO(acac)2, as reference EPA (46) and are listed in Table VII. These values have proved useful for the interpretation of numerous reactions involving complex formation between transition metal ions and halide or pseudohalide ions... 

The potentiometric detector operates on the same principles as ion-selective electrodes. An indicating electrode measures a change in the potential in the presence of certain sample ions. Schmuckler et al. [47] explored the use of potenliometric-type detectors for ion chromatography. Halides and pseudohalides were detected with a sil-ver/silver salt indicator electrode. Other indicating electrodes were also suggested, for example, lead/lead salicylate to detect sulfate type anions. Other workers have reported potentiometric detection [48-52]. 

Halide and pseudohalide complexes.26 In the halide systems, depending on the concentration of halide ions, there are equilibria such as27 HgX+ HgX2 HgXJ HgXJ-... 

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