Outer sphere coordination solvation

The importance of the carboxylate donors is underlined by a study of the lanthanide coordination chemistry of the similar terdentate ligand 2,6 -bis( 1 -pyrazol-3 -yl)pyridine, L24 (63). The complex structure of [Tb(L24)3][PF6]3, shown in Fig. 11, appears to be fairly robust in methanolic solution, with Horrocks analysis (q = 0.6) suggesting the 9-coordinate structure is retained the small quenching effect of outer sphere coordination explains the q-value. However, in aqueous solution, the lability of the ligands dramatically changes the luminescence. Whilst the emission decays are not exactly single exponential, approximate lifetimes in H20 and DoO suggest a solvation value of 4-5. 

Whilst such a classification [1, 9] is useful because it indicates the different types of chemical changes which can occur at the metal centres during phase transfer, it oversimplifies the situation in many cases and fails to indicate the importance of the outer sphere coordination chemistry and of solvation effects in general on the free energies of extraction. Alternative classifications of extraction processes which take better account of these have been presented recently by the Moyer Group. [22] The importance of such supramolecular effects in the design of reagents will be stressed in the examples below. 

In general, two distinct types of processes have been recognized, due to the fact that the ions in solution are almost always surrounded by two layers of solvent molecules. In particular, in the case of a transition element such as cobalt, the first hydration sphere or inner shell corresponds to ligands fixed by coordination bonds and the second, outer sphere includes solvation molecules fixed less rigidly by electrostatic attraction. If, in the active complex, only the outer spheres overlap, the process takes place by the outer sphere , but if the inner spheres overlap, it takes place by the inner coordination sphere . 

Figure 1 depicts schematically the interaction of solvated ions with the electrode surface both for outer sphere (a) and inner sphere pathways (b). Notice, however, that some ambiguity is found with respect to the previous definitions for case (c) in which the coordination sphere of the reacting ion penetrates the layer of solvent molecules adjacent to the electrode, but the ligand is the same solvent molecule and therefore cannot be distinguished from the inner layer of solvent molecules. This may be considered an outer sphere pathway unless the solvent ligand adjacent to the electrode is not present in the product of reaction. 

Other types of complexes in aqueous as well as in nonaqueous solutions can also be successfully studied by means of X-ray diffraction and their structures can often be derived, completely or in part (7,12-15). Structural changes in the coordination sphere of a metal ion, caused by the stepwise replacement of solvent molecules on addition of a ligand, can be followed. Under favorable conditions structures can be derived for the whole series of complexes formed, from the solvated noncom-plexed metal ion to the complex in which all solvent molecules have been replaced. Structural changes beyond the first coordination sphere can sometimes be determined, making it possible to differentiate between inner- and outer-sphere complexes. Even when the information that can be obtained from X-ray diffraction measurements is not sufficient for an unambiguous determination of a structure, it may still give significant structural features, which, in combination with information from other sources, can lead to the complete structure. 

It is conventional to classify electrochemical reactions as outer-sphere and inner-sphere. The former involve the outer coordination sphere of a reacting ion. Thus, little if any change inside the ion solvate shell occurs they proceed without breaking-up intramolecular bonds. But in the latter, involving the inner coordination sphere, electron transfer is accompanied by breaking up or formation of such bonds. Often the inner-sphere reactions are complicated by adsorption of reactants and/or reaction products on the electrode surface. The electron transfer in the Fc(CN)62 /4 system is example of an outer-sphere reaction (with due reservation for some complications... 

In 1954 Weiss32 used Bernal and Fowler s simplified solvation model,16 with an Inner Sphere of ionic coordination, i.e., a small spherical double layer around the ion of charge ze, followed by a sharp discontinuity at radius q, the edge of the Outer Sphere or Dielectric Continuum. He used a simple electrostatic argument to determine the energy to remove an electron at optical frequency from the Inner Sphere ... 

Both AP and Ga have a tightly bound hydrate shell in aqueous solution and both are prone to hydrolysis. In terms of the Hertz electrostatic model for quadrupolar relaxation of ionic nuclei in electrolyte solution (see Section III.C) one therefore expects effective quenching of the electric field gradient caused by the surrounding water dipoles, due to a nearly perfect coordination symmetry. Any contribution to the e.f.g. should therefore arise from outer-sphere solvent dipoles. In terms of the fully orientated solvation (FOS) model this would correspond to a distribution width parameter approaching zero (/. -> 0) with the first term in equation (4) vanishing. This is indeed what Hertz (24) found for both AF" and Ga ", and the experimental infinite dilution relaxation rates ( AP" 7-5 s Ga 350 s ) are remarkably well matched by the computed ones... 

Lanthanide halides, nitrates and triflates are not only common reagents in organic synthesis (Fig. 1) but also represent, in dehydrated form, key precursor compounds for the more reactive organometallics (Scheme 2). As a rule, in compounds of strong monobasic acids or even superacids, cation solvation competes with anion complexation, which is revealed by fully or partially separated anions and solvated cations in their solid state structures. The tendency to form outer sphere complexation in coordinating solvents [47] is used as a criterion of the reactivity of inorganic salt precursors in organometallic transformations. 

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