Counterion-exchange reaction

The equilibrium reaction between neutral hexacoordinate chelates and pentacoordinate siliconium halide salts is discussed in Section III.A.4 (Eq. 17). This reaction can be driven completely to the ionic side by replacement of the chloro ligand by better leaving groups triflate and bromide (Eq. (18), listed again with compound labels see Section III.A.4.iv). The products of this counterion-exchange reaction are stable siliconium salts 90(OTf)-92(OTf), 90(Br)-92(Br), which no... 

M. D. Paulsen, C. F. Anderson, and M. T. Record Jr., Biopolymers, 27, 1249 (1988). Counterion Exchange Reactions on DNA Monte Carlo and Poisson-Boltzmann Analysis. 

Mixing oppositely charged polyelectrolytes leads to the spontaneous formation of polyelectrolyte complexes concomitant with the release of counterions (Fig. la-c). As a first approximation, such a polyelectrolyte complexation may be described as a counterion exchange reaction in a system of polyanions (PA), polycations (PC), counterions (c", c ) and solvent [23, 24], according to ... 

Fig. 1 Schematic illustrations of effects of chain flexibilities on stmctural motifs and associated entropic effects occurring on polyelectrolyte complex formation. Complexation of flexible polyanions and polycations a may lead to non-stoichometric release of counterions being an important contribution to the increased entropy associated with PEC formation. Complexation of semiflexible polyanion and flexible polycation may lead to non-stochiometric release of counterions relative to the overall valence of the polycation (b). Complexation between semiflexible polyanions and polycations may yield a near stoichiometric release of counterions associated with the counterion exchange reaction in an idealized ladder-lUse structure (c). The associated structures are often referred to as d scrambled egg and e railway track structural motifs for the flexible and inflexible, respectively, pairs of interacting polymers. Panels d and e are reproduced from ref [25]with permission from John Wiley and Sons...

The above statements are valid for monomolecular layers only. In the case of polymer films with layer thickness into the p-range, as are usually produced by electropolymerization, account must also be taken of the fact that the charge transport is dependent on both the electron exchange reactions between neighbouring oxidized and reduced sites and the flux of counterions in keeping with the principle of electroneutrality Although the molecular mechanisms of these processes... 

One of the first significant advances in the chemistry of TT-allylpalladium complexes was the discovery that alkenes could be directly converted into the corresponding allyl complex by substitution into the allylic C—H bond. A variety of recipes have now been reported that can accomplish this transformation. Initially, palladium chloride17-23 or its more soluble forms, sodium or lithium tetrachloropalladate24-27 and bisacetonitrile palladium dichloride,28-30 in alcohol or aqueous acetic acid solvent were employed. The use of palladium trifluoroacetate, followed by counterion exchange with chloride, represents the mildest and most effective means available to accomplish this reaction.31... 

Most of the redox centers in a polymer film cannot rapidly come into direct contact with the electrode surface. The widely accepted mechanism proposed for electron transport is one in which the electroactive sites become oxidized or reduced by a succession of electron-transfer self-exchange reactions between neighboring redox sites [22]. However, control of the overall rate is a more complex problem. To maintain electroneutrality within the film, a flow of counterions and associated solvent is necessary during electron transport. There is also motion of the polymer chains and the attached redox centers which provides an additional diffusive process for transport. The rate-determining step in the electron site-site hopping is still in question and is likely to be different in different materials. 

Metal-14 anions react with alkyl halides (RX) mostly by nucleophilic substitution (Sn2), the stereochemistry of which is dependent on the structure of R and X, the solvent and the nature of the counterion. Other reactions were also observed nucleophilic substitution at halogen [also called halogen/metal exchange (HME)] and single electron processes. In some cases steric hindrance around the reactant results in elimination. 

The anion-exchange reaction of the counterion in viologens is performed from the corresponding halides using an anion-exchange resin loaded with a given anion. 

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