Cation counter anions

Table 11.17 Relative Selectivity of Various Counter Cations Table 11.18 Relative Selectivity of Various Counter Anions...

Fig. 1 Relative energy comparison between neutral and solvent coordinated cationic species, associated with counter anion CF [A], Br [B], and I [C].

An ion-selective electrode contains a semipermeable membrane in contact with a reference solution on one side and a sample solution on the other side. The membrane will be permeable to either cations or anions and the transport of counter ions will be restricted by the membrane, and thus a separation of charge occurs at the interface. This is the Donnan potential (Fig. 5 a) and contains the analytically useful information. A concentration gradient will promote diffusion of ions within the membrane. If the ionic mobilities vary greatly, a charge separation occurs (Fig. 5 b) giving rise to what is called a diffusion potential. 

It is not clear whether the X anion remains ligated to the palladium(II) center. For example, for acetic acid, the palladium hydride was initially postulated as being HPd(OAc)L ,377,378 but more recently as HPdL +.367 To date, none of these complexes has been characterized.367 Oxidative addition of acetic acid or formic acid to a palladium(O) complex in DMF affords a cationic palladium hydride /ruw.v-I IPd(PPh3)2(DMF)+, with an acetate or a formate counter-anion. Both reactions are reversible and involve an unfavorable equilibrium so that a large excess of acid is required for the quantitative formation of the palladium hydride complex.379 This allows us to conclude that the catalytic reactions initiated by reaction of palladium(O) and acetic acid (or formic acid) proceed via a cationic palladium hydride trans-HPdfPPtHWDMF)"1", when they are performed in DMF.379... 

It should be noted that the three-dimensional polyether cages (the cryptands) are usually most effective at producing naked anions . With these, the metal ion is completely encapsulated by the polyether network and thus better charge separation is achieved. In the case of the crowns, such complete encapsulation does not normally occur and hence the counter anion is more readily able to associate directly with the com-plexed metal cation. In such cases, the use of the term naked is somewhat of a misnomer. 

Both the diradical and the CF3 derivative are monomeric in the solid state,94 affording materials in which layers of radical cations are sandwiched between layers of the corresponding counter anions. 

The active species generated when bis(arylimino)pyridine iron (5) and cobalt (6) halides are activated with MAO was, by analogy with metallocene catalysts, initially considered to be a highly reactive mono-methylated cobalt(II) or iron(II) cation of the form LM-Me+ bearing a weakly coordinating counter-anion such as [X-MAO]-(X = halide, Me). To examine this theory a number of spectroscopic investigations have been directed towards identifying the active species (vide infra). 

Cationic 3-oxobutylidenaminatocobalt complexes (544a), with hexafluoroan-timonate as a counter anion, activate a, ft-un saturated aldehydes in 1,3-dipolar... 

Fig. 3 View of the cationic chain in [Cu(hyptrz)3](4-chloro-3-nitrophenylsulfonate)2-H20. Counter anions and water molecules have been omitted for clarity. Shitting black, white, and hatched small spheres correspond to nitrogen, carbon and oxygen atoms, respectively. The larger black spheres correspond to copper(II) ions...

The influence of pressure has also been used to tune the ST properties of these ID chain compounds. Application of hydrostatic pressure ( 6 kbar) on [Fe(hyptrz)3] (4-chlorophenylsulfonate)2 H20 (hyptrz=4-(3 -hydroxypro-pyl)-l,2,4-triazole) provokes a parallel shift of the ST curves upwards to room temperature (Fig. 5) [41]. The steepness of the ST curves along with the hysteresis width remain practically constant. This lends support to the assertion that cooperative interactions are confined within the Fe(II) triazole chain. Thus a change in external pressure has an effect on the SCO behaviour comparable to a change in internal electrostatic pressure due to anion-cation interactions (e.g. changing the counter-anion). Both lead to considerable shifts in transition temperatures without significant influence on the hysteresis width. Several theoretical models have been developed to predict such SCO behaviour of ID chain compounds under pressure [50-52]. Figure 5 (bottom) also shows the pressure dependence of the LS fraction, yLS, of... 

Figure 27 Schematic representation of the orientation of 4,4 -bipyridinium radical cations in (a)HV2+/AA and (b) AV2+/AA LB films. Counter anions (TFPB ) and AA are not shown for simplicity.

As the range of components available for use in the azoic dyeing process expanded, research was simultaneously targeted on improvements designed to make the process more attractive to the commercial dyer. The necessity for the dyer to diazotise the Fast Base was removed with the introduction of stabilised diazonium salts [111], known as Fast Salts. Stabilisation was achieved by a judicious selection of the counter-ion to the diazonium cation various anions have found use in commercial Fast Salts and some examples are listed in Table 4-4. Particularly effective is the diazonium tetrachlorozincate, which can be readily prepared by adding an excess of zinc chloride solution to a solution of the diazonium salt. The precipitated complex diazonium salt is usually admixed with an inert diluent, which enhances its stability, and in use the dyer only needs to dissolve the powder in water to prepare the necessary diazonium salt solution. 

The cation pool method is based on the irreversible oxidative generation of organic cations. In the first step, the cation precursor is oxidized via an electrochemical method. An organic cation thus generated is accumulated in the solution in the absence of a nucleophile that we want to introduce onto the cationic carbon. Counter anions which are normally considered to be very weak nucleophiles are used to avoid the nucleophilic attack on the cationic center. In order to avoid thermal decomposition of the cation, electrolysis should be carried out at low temperatures such as -78 °C. After electrolysis is complete, the nucleophile is then added to obtain the desired product. The use of a carbon nucleophile results the direct carbon-carbon bond formation. 

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