Inner sphere complexes mechanisms

The mechanism given is in support of the existence of inner-sphere surface complexes it illustrates that one of the water molecules coordinated to the metal ion has to dissociate in order to form an inner-sphere complex if this H20-loss is slow, then the adsorption, i.e., the binding of the metal ion to the surface ligands, is slow. 

Rates of ligand exchange depend quite strongly on the coordina-tive environment of the metal center. The water exchange rate of Fe(H2O)5(OH)is almost three orders of magnitude higher than that of Fe(H20)g+, and follows a dissociative, rather than an associative exchange mechanism (20). Fe(1120)5(OH)has also been shown to form inner-sphere complexes with phenols (27), catechols (28), and a-hydroxycarboxylic acids (29) much more quickly than Fe(H20) +. The mechanism for complex formation with phenolate anion (A-) is shown below (27) ... 

Complexation reactions are assumed to proceed by a mechanism that involves initial formation of a species in which the cation and the ligand (anion) are separated by one or more intervening molecules of water. The expulsion of this water leads to the formation of the inner sphere complex, in which the anion and cation are in direct contact. Some ligands cannot displace the water and complexation terminates with the formation of the outer sphere species, in which the cation and anion are separated by a molecule of water. Metal cations have been found to form stable inner and outer sphere complexes and for some ligands both forms of complexes may be present simultaneously. 

This account is concerned with the rate and mechanism of the important group of reactions involving metal complex formation. Since the bulk of the studies have been performed in aqueous solution, the reaction will generally refer, specifically, to the replacement of water in the coordination sphere of the metal ion, usually octahedral, by another ligand. The participation of outer sphere complexes (ion pair formation) as intermediates in the formation of inner sphere complexes has been considered for some time (122). Thermodynamic, and kinetic studies of the slowly reacting cobalt(III) and chromium(III) complexes (45, 122) indicate active participation of outer sphere complexes. However, the role of outer sphere complexes in the reactions of labile metal complexes and their general importance in complex formation (33, 34, 41, 111) had to await modern techniques for the study of very rapid reactions. Little evidence has appeared so far for direct participation of the... 

A reaction rate law for the Eigen-Wilkins-Werner mechanism is developed in Section 1.5 (Eqs. 1.50, 1.52, 1.54a, 1.54c). If inner-sphere complex formation is rate limiting and the concentration of water remains constant, the rate of inner-sphere complex formation is (cf. Eq. 1,57)... 

Catalysis in such a mechanism can be attributed to weakening of the peroxidic linkage by formation of an inner-sphere complex with the metal. It could be especially applicable to compounds of the main group elements. Such processes are probably involved in the catalytic decomposition of hydroperoxides by sul-fonium compounds (see later), boron esters, or Se02, e.g.,... 

In Table 4 there are presented data, concerning the specific adsorption of the ions at the silica/aqueous electrolyte interface. Metal ions may adsorb with formation of inner-sphere complexes, hydrocarbon clusters (for example Cu2+) or outer-spherical complexes as for example Mn2+ [105]. The determination of the adequate adsorption mechanism is possible with in situ spectroscopy method (Table 4). 

The net reaction for the reduction of Pu(VI) to Pu(V) by Fe(II) is quite simple in spite of this a complicated three-term hydrogen ion dependence was found (56). A mechanism which involves both outer-sphere and inner-sphere activated complexes is favored. The inner-sphere complexes are supported by evidence for consecutive reactions and a binuclear intermediate. 

Direct initiation by lower valence states (M" ] of metals proceeds through formation of activated complexes with O2 (23, 45)—mostly via inner sphere complexes. As free reduced metals react rapidly with oxygen (Reaction 6a), this mechanism is active primarily when chelators specifically stabilize the reduced metals. These reactions also proceed mostly facilely in nonpolar solvent (46), e.g., in hydrophobic lipid phases of membranes or in oils. 

There are two widely accepted mechanisms for adsorption of solutes by a solid surface. Outer-sphere surface complexation, or non-specific adsorption, involves the electrostatic attraction between a charged surface and an oppositely charged ion in solution (Fig. 3). The adsorbed ion resides at a certain distance from the mineral surface. Inner-sphere complexation, also termed specific adsorption, involves the formation of a coordinative complex with the mineral surface (Kingston et al., 1972 Fig. 3). Inner-sphere complex bonds are more difficult to break than outer-sphere complex bonds and result in stronger adsorption of ions. 

As shown by Tunesi and Anderson (48), the efficiency of the photoredox nrocess for organic compounds depends on their adsorption behavior. When direct charge transfer (inner-sphere complexes) occurs, this mechanism is more efficient than free radical attack. These authors interpret their results nth salicylate at low pH as a direct electron transfer from the adsorbed organic molecule—assumed to be an orbital configuration of the chelate ring— the semiconductor. 

In a later, more detailed study, the outer-sphere complexation of hexahydrated Mil" by FA is confirmed by ascribing the H relaxation mechanism mainly to distortion of the octahedral hydrated-metal symmetry occurring by collisions with water molecules outside the complex (Deczky and Langford, 1978). In the presence of FA, the marked increase observed for the longitudinal relaxation rate dependence on Cu ion concentration compared to Cu -aquo ion and Cu " "-bipyridine complex is attributed to an inner-sphere complex of Cu " " with FA (Deczky and Langford, 1978). 

The sorption mechanism of chromate is unclear. Zachara et al. (1989) suggested that chromate forms an outer-sphere complex on the surfaces of Fe and Al oxides. However, spectroscopic studies have shown that chromate forms inner-sphere complexes (both bidentate and monodentate) on goethite (Fendorf et al., 1997). This anion has a smaller shared charge than do arsenite and arsenate. 

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