Outer sphere cationic shell

Figure 3.6.1 Energies of outer-sphere shell formation kj/mol) against the number of outer-sphere cations for particles (a) nM+Nb E/ and (b) nM Nb E/ ( ) Na, (2) K and (3) Cs...

Examination of the structures of Ln(III) hydrates in crystals and our knowledge of Ln(III) complexes in solution now throws up a problem which the above equations do not readily meet. There is no certain distinction between inner and outer sphere for ions such as Ln(III). Firstly the inner sphere is constantly switching between 8- and 9-coordination but 9-coordination is not far from 6-innermost water molecules which can distort to an octahedron and 3-outermost water molecules. The steps of kinetics can involve multiple re-arrangements of the cation hydration shell which is itself variable in the series of Ln(III). The model equations above are only guides to thinking. 

During the exchange in the interlayer space, the cations keep their hydrate shell this process is called outer-sphere complexation. It is directed by electrostatic forces that, the greater the charge and smaller the size of the hydrated cation, the more favorable is the ion exchange. 

The negative layer charge is mostly neutralized by the hydrated cations in the interlayer space. These cations are bonded to the internal surfaces by electrostatic forces, and they are exchangeable with other cations. The interaction strength between the hydrated cation and the layers (the internal surface) increases when the charge of the cation increases, and the hydrated ionic radius decreases. Cations with hydrate shell can be considered as outer-sphere complexes. Cation exchange is the determining interfacial process of the internal surfaces of montmorillonite. 

As Table 6.3 shows, this classification into A- and B-type metal cations is governed by the number of electrons in the outer shell. A-type metal cations having the inert gas type d electron configuration correspond to those that were classified above as hard sphere cations. These ions may be visualized... 

Cations adsorbed on the basal planes of smectites can be immobilized in two kinds of surface complex (4). The surface complex is inner-sphere if the cation is bound directly to a cluster of surface oxygen ions, with no water molecules interposed, and it is outer-sphere if one or more water molecules is interposed between the cation and the siloxane surface to which it binds 30). Thus, adsorbed cations in outer-sphere surface complexes retain solvation shells, whereas those in inner-sphere surface complexes can at most be only partially solvated. Spectroscopic data 31) suggest that monovalent cations in inner-sphere surface complexes remain immobilized on a timescale of ca. 100 ps, whereas those in outer-sphere surface complexes are able to diffuse along the siloxane surface over this timescale. 

In describing the properties of complex species in alkali metal halide melts and the electrochemical processes involving these species, the anionic complex should be considered integral with its outer-sphere (OS) cationic shell [1-4], In model calculations, the composition of this shell is chosen rather arbitrarily. However, calculations show that, in many cases, variations in the composition of the second coordination sphere in the model system radically changes the resulting correlations. Therefore, the task is to search for criteria that permit determining the composition of the dominant complex species in alkali metal halide. 

Step 12 involves diffusion-controlled outer-sphere complex formation, followed by the loss of bound solvent from the ligand solvation shell to form a second outer-sphere complex. Step 111 is the loss of solvent from the cation solvation shell followed by the cation-ligand bond formation. If a change in coordination number occurs during the complexation reaction, it is most often coupled to step 111. 

Here OH2 represents the shells of polarised water molecules surrounding the ions. In the first reaction the sulphate ion loses some of its closely adhering water and an outer sphere ion-pair is formed in the second reaction the intervening water is all lost and an inner sphere or contact ion-pair is formed. It is the sum of these two types of ion-pair that is measured by the fall in the molar conductivity or activity, and their relative amounts depend on how much short-range forces between the ions favour the stability of the contact ion-pair. Another hydration effect is found in the salts of the oxy-acids, which tend to show more ion-pairing than other salts of the same valence type. Here there is some evidence that the oxygen atoms of the nitrate ion, for example, readily take the place of water molecules in the hydration shell of the cation, so that contact ion-pairs are more readily formed. Finally, another factor is illustrated by the dissociation constants of the amino-acetates, which vary by more than a millionfold in... 

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