Ion pairing between cations and

These observations are consistent with the reactive species being constituted from tight ion pairs between cations and the alkoxide anions resulting from abstraction of hydrogen atoms in A, B and C (Scheme 3.11). 

Alkali-metal cation catalysis of electron transfer to Acceptor-anions is generally attributed (1) to the formation of stable association complexes (ion pairs) between cation and Acceptor-anion or (2) to formation of stable ternary association complexes that additionally include the Donor as well. ° (These are distinct from the ternary activated complexes formed later along the reaction coordinate.) In case (1), increase in kobs for electron transfer is often attributed to a positive shift in the reduction potential of the acceptor complex upon cation association. Such positive shifts in reduction potential are... 

Ion pairing between cations and anions is a barrier for the inclusion of cations into the pillar[5]arene cavity because strong ion-pairing prevents the complexation. Wang and co-workers synthesized a heteroditopic pillar[5]-arene containing one urea moiety (H5.15) for ion-pair recognition of alky-lammonium cations and counter anions (Figure 5.20). ... 

As Fig. 2.4 illustrates, a cation can associate with a surface as an inner sphere, or outer-sphere complex depending on whether a chemical, i.e., a largely covalent bond, between the metal and the electron donating oxygen ions, is formed (as in an inner-sphere type solute complex) or if a cation of opposite charge approaches the surface groups within a critical distance as with solute ion pairs the cation and the base are separated by one (or more) water molecules. Furthermore, ions may be in the diffuse swarm of the double layer. 

Fig. 2.6 Ion pairs. A two-dimensional representation of (a) a solvent-separated pair of ions, each still retaining its intact shell of solvating solvent molecules (b) a solventsharing ion pair, which has lost some of the solvent between the partners, so that one layer of solvent shared between them separates them (c) a contact ion pair, the cation and anion being contiguous.

In solvent-separated ion pairs, the solvation shells of the cation and the anion touch each other in solvent-bridged ion pairs, the ions share solvent molecules. In contact ion pairs, the cation and the anion are bound directly to each other and are surrounded by a common solvation shell. In penetrated ion pairs, an empty space between edge groups in one ion of a salt is occupied to a certain degree by a counterion. The two latter types of ion pair may have quite a different electronic distribution than the corresponding naked ions. The following examples show the influence of ion-pair formation. 

Sodium diethyldithiocarbamate (DDTC) was used to separate transition metal cation with a CTAB micellar phase and a CIS column [32]. The limits of detection obtained with atomic absorption spectroscopy were in the tens of picograms injected. Since a high concentration of i-propanol (45% v/v) was added to the 0.03 M CTAB mobile phase, the presence of micelles may be discussed. Simple ion-pairing between CTAB and the DDTC metal species may explain the observed selectivity. Tartaric acid was also used as a ligand for transition metal cations with a SDS micellar mobile phase and a CIS column [33]. 

These 2,2 -bipyrimidine complexes are relatively unstable, and attack by azide and thiocyanate, not established for, e.g., the [Fe(bipy)3] cation, has been observed for the [Fe(bipym)3] + cation. The ferrozine [fz=(3)] complex [Fe(fz)s] " is very stable and one of the most inert di-imine-iron(ii) complexes. There is here ample time to demonstrate the rapid formation of an intermediate, and to follow the decay of the [Fe(fz)3] " and the intermediate by conventional spectroscopic techniques. The use of aqueous methanol as solvent proved advantageous, as this increased the chemical potential of the hydroxide ion and thence increased formation of the intermediate. The 4— charge on the complex here, in contrast to the great majority of di-imine complexes studied, rules out significant ion-pairing between complex and hydroxide. ... 

In nonpolar solvents, anionic [Pd L Cl] are probably not formed due to ion-pairing between Cl and the cation m deUveied by the nucleophile. Consequently, the formation of the anionic [PhPdI(Cl)LJ can be also bypassed. frans-PhPdILj is thus formed and reacts with the nucleophile (neutral pathway in Scheme 19.28). The cross-coupling product is however delivered in a slower reaction because it is retarded by the endergonic transicis equilibrium. 

Photoamination of 6 proceeds via the nucleophilic addition to the radical ion pairs between 6 and m-DCB followed by one-electron reduction of the aminated radicals with m-DCB" and protonation to give 26 according to Scheme 6.8. However, the highly reactive localized cation radicals of 6, in general, tends to cause side reactions involving dimerization, deprotonation, and isomerization, resulting in the amination in lower yields. It is well known that aromatic hydrocarbons (ArH) work as tr-donor that can interact with a cation radical [49]. The additive effect of 1,3,5-TPB or m-TP appears to come from the formation of Ji-complex with 6". The Jt-complex formation with ArH would lower the reactivity of the cation radicals and suppress the side reactions, resulting in the effective photoamination of 6. 

Atomic absorption spectrometry (AAS) has been used to determine cationic and anionic surfactants indirectly. Two methods have been put forward based on the formation of the ion pair between surfactant and hexanitrocobaltate (for cationic compounds) or bis(benzoyl)pyridine thiosemicarbazone cobalt (III) (for anionic compounds). In the former case, the complex is extracted with 1,3-dicloroethane and in the latter with an isopentylacetate and isopentyl alcohol mixture. Concentration of cobalt is determined in the organic phase using electrothermal atomic absorption spectroscopy (ETAAS), while for anionic surfactants, flame atomic absorption spectroscopy (FAAS) can also be used. Interferences like metal ions, anions and organic compounds do not have a great relevance. The two methods were applied to determine dodecyltrimethylammonium bromide in shampoos (Chattaraj and Das, 1992) and sodium lauryl sulfate (SDS) in toothpastes (Chattaraj and Das, 1994). 

The sensor layer consists of a selective ionophore (e.g. valinomycin for potassium), a lipophilic anionic site (borate) and the cationic PSD. Before interaction with potassium, a lipophilic ion pair between the cationic PSD and borate anion is formed in the polymer layer. When valinomycin (also contained in the layer) selectively extracts potassium into the layer, then the positively charged valinomycin-potassium complex forms an ion pair with... 

The proportion of C-alkylation increases in the order OTs < Br < I, a sequence which is often associated with the balance of hardness between nucleophile and nucleofuge (Smith and Hanson, 1971). The work of Kurts et al. (1974) indicates that the overall reaction rate of the crown ether-assisted alkylation increases in the order Na+ < K+ < Rb+ < Cs+, which, according to these authors, reflects the increasing distance between cation and anion in the ion pairs. The high reactivity of the tetraphenylarsenate also fits in with this picture. The decrease of the kc/k0 ratio is only small in good cation-solvating solvents such as dimethyl sulfoxide (DMSO). Alkylation of the sodium derivative of [103] with ethyl iodide in DMSO gave kc/kQ = 15.7 addition of... 

Ion pairing Electrostatic attractions that arise between cations and anions in solutions of high ionic strength. These attractions are not as strong as ionic bonds. Nevertheless, their presence can influence the physical and chemical behavior of the solution. 

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