Inner-sphere ion pair

Infra-red spectra of charge-transfer salts as contact (inner-sphere) ion pairs in the solid state and in solution... 

Surface complexation models attempt to represent on a molecular level realistic surface complexes e.g., models attempt to distinguish between inner- or outer-sphere surface complexes, i.e., those that lose portions of or retain their primary hydration sheath, respectively, in forming surface complexes. The type of bonding is also used to characterize different types of surface complexes e.g., a distinction between coordinative (sharing of electrons) or ionic bonding is often made. While surface coordination complexes are always inner-sphere, ion-pair complexes can be either inner- or outer-sphere. Representing model analogues to surface complexes has two parts stoichiometry and closeness of approach of metal ion to... 

Although the cation-anion interaction of metallocenium ions is very weak, the counteranion is likely to remain in proximity with the metal cation to form a contact ion pair in low-permittivity solvents such as toluene (commonly used in polymerization reactions). If the metal cation allows the counteranion to penetrate into the first coordination sphere, it can form an inner-sphere ion pair (ISIP). When the counteranion is relegated to the second coordinating sphere, the ion pair becomes an outer-sphere ion pair (OSIP), which is more ionic in nature than ISIPs. A schematic representation of the relationship between ISIPs and OSIPs is depicted in Scheme 2. This simple scheme shows us the principal elements that affect the cation-anion interactions (e.g., counteranion (Y ), ancillary ligands (L ), transition metal (M), and alkyl ligand (R)), and the subtle balance between these elements in the dynamic equilibria. 

Iron thio-terephthalates, for fungus and molds, 12, 458 Iron veroheme, reactivity, 6, 106-107 ISIPs, see Inner-sphere ion pairs... 

It should be mentioned that Marcus [5] uses the terms inner-sphere ion pair and outher-sphere ion pair for CIP and SSIP, respectively. Depending on the degree of penetration of the solvation shells one may further differentiate between SSIP and solvent shared ion pairs . However, a clear experimental assignment has not yet been performed. Therefore, we will use the designation CIP and SSIP... 

The visible spectmm picks up inner-sphere ion pairs and complexes. 

SlYOKA AM] Yokoyama, H., Yamatera, H., Ion association of some 2 2 electrolytes in water at 25 C. III. A new interpretation of experimental result and the determination of formation constants of inner-sphere ion-pairs. Bull. Chem. Soc. Jpn., 54, (1981), 2286-2289. Cited on pages 184, 397. 

As argued by Colhns, inner sphere ion pairs are preferentially formed between oppositely charged ions with matching absolute enthalpies of hydration. 

Highly active and more well-defined catalyst systems have become accessible by reaction of a zirconocene alkyl precursor with one of several cationization reagents that contain a trityl or dimethylanilinum cation capable of abstracting an alkyl anion from this precursor. An inert tetra(perfluorophenyl) borate or related anion coordinates only weakly to the resulting alkyl zirconocenium cation in an inner-sphere ion pair of type A (Fig. 3) [26, 28, 29]. 

In MAO-activated catalyst systems, alkyl zirconocenium cations are likewise thought to be present, presumably in weakly botmd inner-sphere ion pairs with anions of the type MeMAO [26, 29]. These anions - still only vaguely characterized as large agglomerates [32] - are assumed to be formed from MAO by uptake of a methyl anion from the alkyl zirconocene precursor. In equilibrium with these inner-sphere ion pairs A, outer-sphere ion pairs B (Fig. 3) are observed in MAO-activated pre-catalyst systems [29, 32-34] that contain a heterobinuclear cationic AlMes adduct [35], presumably together with MeMAO as counter-anion. 

In the previous sections we repeatedly mentioned the general rule like seeks like . But what is the reason Kim D. Collins published a landmark paper to elucidate and explain this phenomenon without using sophisticated theories. We quote here his main idea that he summarised in the following rule The law of matching water affinities oppositely charged ions in free solution form inner sphere ion pairs spontaneously only when they have equal water affinities. 

Fig. 17. Ion size controls the tendency of oppositely charged ions to form inner sphere ion pairs. Small ions of opposite sign spontaneously form inner sphere ion pairs in aqueous solution large ions of opposite sign spontaneously form inner sphere ion pairs in aqueous solution and mismatched ions of opposite sign do not spontaneously form inner sphere ion pairs in aqueous solution. A large monovalent cation has a radius larger than 1.06 A a large monovalent anion has a radius larger than 1.78 A. (After CoUins.30)...

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