Cation-anion attractions

Binding interactions available to these ions are essentially electrostatic in nature, namely, cation-anion attractions responsible for cation-pairing to counter-anions, and ion-dipole interactions that provide the basis for the well-known complexation with crown ethers. 

Bochmann has also produced a novel class of large dianionic borates XV based on the non-labile fefra-cyano Ni and Pd(II) metallates [K12 M(CN) i 2. 215a The metals in these WCAs are strictly square planar and as such the anion assumes a flat rather than spherical shape, with a torus of fluorination on the periphery of the plane. The bis-trityl salts of these dianions afford catalysts that are not as productive as those formed from the XIV class (X = CN), although they are still much more active than borane activated catalysts. The lower activity was attributed to the inherently greater cation/anion attraction in a monocation/dianion pair. 

The formation of complex ions is the result of cation-anion attractive forces winning out in the competition between cations and H+ for the various ligands, including water. An example is the formation of the monofluoroaluminium complex ion... 

Because ion pairs are formed in low polarity aprotic solvents, the anion of organic cations also contributes to the complexation. It has been found that the stability of complexes between aromatic receptors and ammonium and iminium ions is decreased in the order picrate>trifluoroacetate>r>Br >Cr>tosylate>acetate. Thus, the lower the cation-anion attraction, the stronger is the cation-rr interaction. ... 

The only interactions the theory considers are the electrostatic interactions between ions. These interactions are much stronger than those between imcharged molecules, and they die off more slowly with distance. If the positions of ions in an electrolyte solution were completely random, the net effect of electrostatic ion-ion interactions would be zero, because each cation-cation or anion-anion repulsion would be balanced by a cation-anion attraction. The positions are not random, however each cation has a surplus of anions in its immediate environmenL and each anion has a surplus of neighboring cations. Each ion therefore has a net attractive interaction with the surrounding ion atmosphere. The result for a cation species at low electrolyte molality is a decrease of /r+ compared to the cation at same molality in the absence of ion-ion interactions, meaning that the single-ion activity coefficient y+ becomes less than 1 as the electrolyte molality is increased beyond the ideal-dilute range. Similarly, y also becomes less than 1. 

The different layer-interlayer-layer sequences that are possible in the chlorite structure create varying amounts of cation-cation repulsion and cation-anion attraction as a result of the different superpositions of sheets. Bailey and Brown [1962] and Shirozu and Bailey [1965] have attempted to explain the observed relative abundances of the chlorite structural types according to the relative stabilities indicated by these interatomic forces. Repulsion between the superposed interlayer and tetrahedral cations in the la and Ila structural units is considered the most important single factor in reducing the stability of these two unit types relative to the lb and lib units. Three other factors considered are ... 

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