Ion-pair formation involving

In addition to proton adsorption, interactions between the ions of the inert electrolyte (counter ions, section 10.3) and the oxide surface lead to ion pair formation which influences the electrochemical properties of the oxides and the determination of pKa values. Ion pair formation involves outer sphere surface complexes (see Chap. 11), e.g. 

Liquid-liquid extraction by ion-pair formation involves formation of an uncharged species as exemplified below for a case in which a tertiary amine like trioctylamine (TOA) or triisooctylamine (TIOA) is used as the solvent to extract the anionic metal complex represented by B. ... 

According to Eigen and Tamm [87,88], ion-pair formation proceeds stepwise, starting from separated solvated ions which form a solvent-separated ion pair [C+SSA ]°, followed by a solvent-shared ion pair [C+SA ]° and finally a contact ion pair, [C+A ]° [Eqs. (4)-(6)]. All these species are solvated. The types of ion pair formed depend on the relative strength of the interaction of the involved species. 

A brief discussion of the systematics of solvent effects on the p/, pr, and values of Tables II and III is presented in the discussion section. However, it is worthy of note here that sets 7, 37, 38, 39, 40, and 41, which involve nonhydroxylic solvents, are fitted with comparable precision to that for reaction series in aqueous or mixed aqueous organic solvents. The present analysis does not support the previous assignment (7b) of ion-pair formation of benzoic acids... 

As discussed already, crowns may be involved in solvent extraction processes in which an inorganic reagent is transferred (sometimes selectively) from one phase (often water) into an immiscible organic phase the extraction involves ion-pair formation between the cationic crown complex and the counter ion (Blasius Janzen, 1981). 

Ion pairs are outer-sphere association complexes, which have to be clearly distinguished from the organometallic complexes discussed in Section 6. Ion pair formation appears to be much less important in biological membranes as compared with octanol, because the charge of the ions at the membrane interphase can be balanced by counter charge in the electrolyte in the adjacent aqueous phase. The reactions involved in ion pair formation are depicted in Figures 5b for acids and 5c for bases, and the equilibrium constant K ix is defined as follows ... 

Throughout the series, ion-pair formation is accompanied by significantly negative AS° values and close-to-zero AV values. The latter is rather surprising, since it is generally accepted that ion-pair formation should involve considerable charge neutralization accompanied... 

Interactions between a solute and a solvent may be broadly divided into three types specific interactions, reaction field and Stark effects, and London-van-der-Waals or dispersion interactions. Specific interactions involve such phenomena as ion pair formation, hydrogen bonding and ir-complexing. Reaction field effects involve the polarization of the surrounding nonpolar solvent by a polar solute molecule resulting in a solvent electric field at the solute molecule. Stark effects involve the polarization of a non-polar solute by polar solvent molecules Dispersion interactions, generally the weakest of the three types, involves nonpolar solutes and nonpolar solvents via snap-shot dipole interactions, etc. For our purposes it is necessary to develop both the qualitative and semiquantita-tive forms in which these kinds of interactions are encountered in studies of solvent effects on coupling constants. 

This account is concerned with the rate and mechanism of the important group of reactions involving metal complex formation. Since the bulk of the studies have been performed in aqueous solution, the reaction will generally refer, specifically, to the replacement of water in the coordination sphere of the metal ion, usually octahedral, by another ligand. The participation of outer sphere complexes (ion pair formation) as intermediates in the formation of inner sphere complexes has been considered for some time (122). Thermodynamic, and kinetic studies of the slowly reacting cobalt(III) and chromium(III) complexes (45, 122) indicate active participation of outer sphere complexes. However, the role of outer sphere complexes in the reactions of labile metal complexes and their general importance in complex formation (33, 34, 41, 111) had to await modern techniques for the study of very rapid reactions. Little evidence has appeared so far for direct participation of the... 

The equilibria considered up to now have all involved inner sphere complexes. There is the possibility that an inner sphere complex may react with free ligands in solution this includes the solvent itself, to give an outer sphere complex where the ligand enters the secondary solvation shell of the inner sphere complex. If the two species involved in this type of interaction are of opposite sign, which is the situation where this type of complex formation is expected to be most effective, the outer sphere complex is called an ion pair. Fuoss65 has derived an expression (equation 38) for the ion pair formation constant, XIP, from electrostatic arguments ... 

Most of the studies involving HA-B combinations were conducted employing such aqueous media, the extraction from which involves the formation of simple ternary adducts MAn yB and their study can easily be carried out with any of the methods given here. But when quartemary adducts or ion-pairs are involved in the extraction it becomes very difficult to establish their stoichiometry by any single method and very often different types of experimental data need be obtained to confirm such species. 

Therefore, thermodynamics plays a fundamental role in supramolecular chemistry. However, thermodynamics is rigorous and as such, a great deal of ancillary information is required prior to the formulation of an equation representative of the process taking place in solution, such as, the composition of the complex and the nature of the speciation in solution. For the latter and when electrolytes are involved, knowledge of the ion-pair formation of the free and complex salts in the appropriate solvent is required particularly in non-aqueous solvents. This information would allow the establishment of the concentrations at which particular ions are the predominant species in solution. Similar considerations must be taken into account when neutral receptors are involved, given that in dipolar aprotic or inert solvents, monomeric species are not always predominant in solution. In addition, awareness of the scope and limitations of the methodology used for the derivation of thermodynamic data for the complexation process is needed and this aspect has been addressed elsewhere [18]. 

Figure 3.28 Illustration of the mechanism of I PC. The solute ion X, the pairing ion Yand the ion-pair XY are all distributed over the two phases. Ion-pair formation occurs in both phases (reactions along horizontal lines). Ion-exchange reactions may also occur. These reactions involve solute ions in one phase and pairing ions in the other. These reactions can be found along diagonal lines in the figure.

---