Distinguishing between inner- and outer-sphere complexes

Nd(III) hypersensitive transitions have been used to distinguish between inner and outer sphere complexes in solutions [203]. 

For this purpose it is possible to extend to a multiple oxide the one-site model of Johnson (1984), which provides a thermodynamic description of the double layer surrounding simple hydrous oxides. Briefly, in this model the double layer charge is divided into the charge inside the slip plane, slip plane a[d]. While occupied sites, a[tl] is obtained from the Poisson-Boltzmann equation. Note that unlike the triple-layer model (Davis et al., 1978) which allows ions to form surface complexes at two different planes (0 or / ) instead of al the slip plane only, this model does not distinguish between inner- and outer-sphere complexes. Expression of the... 

For labile complexes, it is often quite difficult to distinguish between inner and outer sphere complexes. To add to this confusion is the fact that formation constants for such labile complexes when determined by optical spectrometry are often lower than those of the same system determined by other means such as potentiometry, solvent extraction, etc. This has led some authors to identify the former as "inner sphere" constants and the latter as "total" constants. However, others have shown that this cannot be correct even if the optical spectrum of the solvated cation and the outer sphere complex is the same (4, 7). Nevertheless, the characterization and knowledge of the formation constants of outer sphere complexes are important as such complexes play a significant role in the Eigen mechanism of the formation of labile complexes (8) This model describes the formation of complexes as following a sequence ... 

How to distinguish between inner- and outer-sphere complexes in aqueous solution -thermodynamics and other criteria. (S. Ahrland) p. 21. 

Several criteria help to distinguish between inner- and outer-sphere complexes, although not always unambiguously. For instance, Choppin (1971) and Choppin and Bertha (1973) have used the thermodynamic A// and A5 parameters. They have assigned, in aqueous solutions, a predominantly outer-sphere character to CL, Br, L, ClOj, NOj, sulfonate and trichloroacetate complexes and an inner-sphere character to F, lOj, SO and acetate complexes. Moreover, these authors have related this ordering to the pKa values of the ligands. On the other hand, a group of Russian authors have postulated that iimer- and outer- sphere complexes may be studied separately by spectrophotometric methods, but this is subject to some doubt (vide infra). 

Reductions of various Co(ni) complexes by Fe(II) have been studied under high pressures . The motivation for performing such experiments resides in the possibility that the volume of activation (AF ), like the entropy of activation, might be a criterion for distinguishing between inner- and outer-sphere reactions. For reactions of the type... 

In some cases, involving inert complexes of chromium(III) and cobalt(III), it has been possible to distinguish unambigously between inner and outer sphere complexes (33,34). In the latter type of complexes, no water of hydration is displaced by the ligand on complex formation. No bonds directly involving the metal are thus broken or formed, nor are any large number of water molecules set free from the hydration shells. If those interpretations are true which have been given above for the... 

Strawn, D. G., and D. L. Sparks. 1999. The use of XAFS to distinguish between inner- and outer-sphere lead adsorption complexes on Montmorillonite. J. Coll. Interface Sci. 216 257-269. 

NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY Nuclear magnetic resonance (NMR) spectroscopy can be applied to aqueous samples and can distinguish between inner- and outer-sphere ion surface complexes. The adsorption behavior of the cations Cs+ and Na" " was studied on the surfaces of silica, boehmite, kaolinite, and illite (Kim and Kirkpatrick, 1997). Cesium was adsorbed both as inner-and outer-sphere surface complexes and in the diffuse layer, while Na was adsorbed only as outer-sphere surface complexes and in the diffuse layer. The adsorbed Na ions were fully hydrated, while the Cs ions had direct contact with the surface oxygen atoms. 

Stragier H, Cross JO, Rehr JJ, Sorensen LB, Bouldin CE, Woicik JC (1992) Diffraction anomalous fine structure A new X-ray structural technique. Phys Rev Lett 69 3064-3067 Strawn DG, Scheidegger AM, Sparks DL (1998) Kinetics and mechanisms of Pb(II) sorption and desorption at the aluminum oxide-water interface. Environ Sci Technol 32 2596-2601 Strawn DG, Sparks DL (1999) The use of XAFS to distinguish between inner- and outer-sphere lead adsorption complexes on montmorillonite. J Colloid Interface Sci 216 257-269 Strawn DG, Sparks DL (2000) Effects of soil organic matter on the kinetics and mechanisms of ( ) sorption and desorption in soil. Soil Sci Soc Am J 64 144-156 Stumm W (1992) Chemistry of the Sohd-Water Interface. John Wiley Sons, Inc, New York... 

Particular use was made of conductivity measurements of cobalt(iii) and platinum(ii) complexes which allowed a facile determination of the number and type of ions present in solution. For example, the compounds Co(NH3) Cl3 would give a monocation and an monoanion (n=4), a dication and two monoanions (n = 5) and a trication and three monoanions (n=6) respectively. In some cases, it was also possible to distinguish chemically between inner and outer sphere chloride by precipitation of the outer sphere species as AgCl. 

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... 

Often, it is difficult to distinguish definitely between inner sphere and outer sphere complexes in the same system. Based on the preceding discussion of the thermodynamic parameters, AH and AS values can be used, with cation, to obtain insight into the outer vs. inner sphere nature of metal complexes. For inner sphere complexation, the hydration sphere is disrupted more extensively and the net entropy and enthalpy changes are usually positive. In outer sphere complexes, the dehydration sphere is less disrupted. The net enthalpy and entropy changes are negative due to the complexation with its decrease in randomness without a compensatory disruption of the hydration spheres. 

There is a lot of examples of outer-sphere electron-transfer reactions occurring in irradiated systems of typical inorganic complexes [188-191]. However, for metallotetrapyrroles such reactions involving the central atom are rather rare. It should be underlined that it is usually not so simple to distinguish between the primary outer-sphere and inner-sphere step, particularly in cases when both lead to the same product and proceed with comparable rates. Moreover, a number of outer-sphere electron-transfer reactions occur as reversible processes with no net chemical change. To solve this problem, techniques of... 

I nations 82 and 83 distinguish between inner-sphere and outer-sphere complexes of a surface bound Pb(II). Actual bond lengths, conformations, and site energies for coordination complexes on a real surface may vary from one site to n not her. The postulated reactions do, however, provide a means of estimating how adsorption density responds in a semiquantitative way to changing medium conditions. See, for example, Schindler and Stumm (1987) for a comprehensive discussion of acid-base and complexation equilibria at oxide-water interfaces. 

Upon reaction with an adsorptive in aqueous solution (which then becomes an adsorbate), surface functional groups can engage in adsorption complexes, which are immobilized molecular entities comprising the adsorbate and the surface functional group to which it is bound closely [18]. A further classification of adsorption complexes can be made into inner-sphere and outer-sphere surface complexes [19]. An inner-sphere surface complex has no water molecule interposed between the surface functional group and the small ion or molecule it binds, whereas an outer-sphere surface complex has at least one such interposed water molecule. Outer-sphere surface complexes always contain solvated adsorbate ions or molecules. Ions adsorbed in surface complexes are to be distinguished from those adsorbed in the diffuse layer [18] because the former species remain immobilized on a clay mineral surface over time scales that are long when compared, e.g., with the 4-10 ps required for a diffusive step by a solvated free ion in aqueous solution [20]. Outer-sphere surface complexes formed in the interlayers of montmorillonite by Ca2+ or Mg2+ are immobile on the molecular time scale... 

Similar to solution complexation, surface complexation can be distinguished between inner-spherical complexes (e.g. phosphate, fluoride, copper), where the ion is directly bound to the surface, and outer-spherical (e g. sodium, chloride) complexes where the ion is covered by a hydration sleeve with the attraction working only electrostatically. The inner-sphere complex is much stronger and not dependent on electrostatic attraction, i.e. a cation can also be sorbed on a positively charged surface (Drever 1997). 

The chromium(III) chromium(VI) complex listed in Table V almost certainly contains octahedrally coordinated chromium(III) and tetrahedral chromium(VI), as in the separate ions Cr(H20)63+ and Cr042 . It is formed rapidly and reversibly on mixing the solutions of these ions, but the actual rate of formation has not been measured, and since the chromium(VI) ion, but not the chromium(III) ion, is known to undergo substitution rapidly, the experiment does not distinguish between the alternative outer- and inner-sphere structures,... 

It is important to distinguish between outer-sphere and inner-sphere complexes. In inner-sphere complexes the surface hydroxyl groups act as a-donor ligands, which increase the electron density of the coordinated metal ion. Cu(II) bound in an inner-sphere complex is a different chemical entity from Cu(II) bound in an outer-sphere complex or present in the diffuse part of the double layer. The inner-spheric Cu(II) has different chemical properties for example, it has a different redox potential with respect to Cu(I), and its equatorial water is expected to exchange faster than that in Cu(II) bound in an outer-sphere complex. As we shall see, the reactivity of a surface is affected, above all, by inner-sphere complexes. 

At the outset, it is useful to distinguish between inner-sphere and outer-sphere electron-transfer reactions at electrodes (Figure 3.6.1). This terminology was adopted from that used to describe electron-transfer reactions of coordination compounds (54). The term outer-sphere denotes a reaction between two species in which the original coordination spheres are maintained in the activated complex [ electron transfer from one primary bond system to another (54)]. In contrast, inner-sphere reactions occur in an activated complex where the ions share a ligand [ electron transfer within a primary bond system (54)]. 

The triple layer model attempts to take into account inner sphere complex formation and electrostatic adsorption simultaneously by considering "specifically adsorbed" ions which are supposed to be maintained very close to the surface, whether it be through the formation of covalent bonds with some surface groups, or of some outer sphere complex. No specific interpretation of the bonding is required, provided one can define a plane of specific adsorption, located a few A from the surface and containing those ions this is called the Stem layer. The theory distinguishes then between three successive parallel layers the surface plane proper, the Stem layer, and the diffuse layer. 

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