Inner-sphere complex defined

Figure 1. An Fe(H20)62+-Fe(HgO)6s+ complex at the traditional inner-sphere contact distance with the inner-sphere complexes (Th symmetry) oriented to give overall S6 symmetry. This geometry is favorable for transfer of an electron between t g-5d atomic orbitals (AO s, which have

There are a number of more loosely defined terms for different types of adsorption that are related to the form of surface complexation. Specifically adsorbed ions are held in inner-sphere complexes whereas non-specifically adsorbed ions are in outer-sphere complexes or the diffuse-ion swarm. Readily exchangeable... 

Inner-sphere/outer-sphere surface complexes (defined as strong surface complexes or inner-sphere complexes, as opposed to weak surface complexes or outer-sphere complexes)... 

Adsorption defines the accumulation of a substance, or material, at tlie interface between a solid surface and a bathing solution (Sparks, 2002). Within the adsorption framework, the individual components are referred to as the adsorbate, the accumulating material at the interface, and the adsorbent, or solid surface (Sparks, 2002). If adsorption occurs and results in the formation of a stable molecular phase at the interface, this entity can be described as a surface complex. Two general surface complexes exist and are described by the configuration geometry of the adsorbate at the adsorbent surface. These are the iiuier-and outer-sphere surface complexes, defined by the presence, or absence, of the hydration sphere of the adsorbate molecule upon interaction. When at least one water molecule of the hydration sphere is retained upon adsorption, the surface complex is referred to as an outer-sphere complex (Sposito, 1984) when an ion or molecule is bound directly to the adsorbent without the presence of the hydration sphere, an inner-sphere complex is formed. 

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. 

Surface complexation reactions are assumed on surface sites, S—OH. The total site density (Ns, mol/m ), has to be defined for the given system. In the constant-capacitance and diffuse-layer models, all surface species are supposed to be inner-sphere complexes, whereas in the triple-layer model, both inner- and outer-sphere complexes are assumed. 

Where solvent exchange controls the formation kinetics, substitution of a ligand for a solvent molecule in a solvated metal ion has commonly been considered to reflect the two-step process illustrated by [7.1]. A mechanism of this type has been termed a dissociative interchange or 7d process. Initially, complexation involves rapid formation of an outer-sphere complex (of ion-ion or ion-dipole nature) which is characterized by the equilibrium constant Kos. In some cases, the value of Kos may be determined experimentally alternatively, it may be estimated from first principles (Margerum, Cayley, Weatherburn Pagenkopf, 1978). The second step is then the conversion of the outer-sphere complex to an inner-sphere one, the formation of which is controlled by the natural rate of solvent exchange on the metal. Solvent exchange may be defined in terms of its characteristic first-order rate constant, kex, whose value varies widely from one metal to the next. 

Since electrophilic and charge-transfer nitrations are both initiated via the same EDA complex and finally lead to the same array of nitration products, we infer that they share the intermediate stages in common. The strength of this inference rests on the variety of aromatic substrates (with widely differing reactivities and distinctive products) to establish the mechanistic criteria by which the identity of the two pathways are exhaustively tested. On this basis, electrophilic nitration is operationally equivalent to charge-transfer nitration in which electron-transfer activation is the obligatory first step. The extent to which the reactive triad in (90) is subject to intermolecu-lar interactions in the first interval (a few picoseconds) following electron transfer will, it is hoped, further define the mechanistic nuances of dissociative electron transfer in adiabatic and vertical systems (Shaik, 1991 Andrieux et al., 1992), especially when inner-sphere pathways are considered (Kochi, 1992). 

In view of the discussion just previous, it is natural to inquire into the circumstances under which the investigation of precursor complexes might lead to an assignment of inner-sphere vs. outer-sphere mechanism. The issue is not independent of the previous discussion because the successor complex for the forward reaction is the precursor complex for the reverse. If the reaction mechanism has been defined for the forward direction, it is defined also for that portion of the reverse reaction which makes use of the same path. But in terms of the experimental criteria which are... 

As has already been acknowledged, classifying a reaction mechanism as inner sphere only advances us part way to the goal of defining the nature of the activated complex. In this section the most thoroughly studied reaction of the inner-sphere class is chosen for description and discussion so as to indicate some of the additional issues which can profitably be addressed. 

An interesting subset of the inner-sphere electron transfer reactions involves the irreversible formation of a stable metal-02 adduct as the product. A series of such reactions (Figure 9.4) has been investigated by reacting d8 and d10 organometallic complexes with O2.45 These reactions result in the formation of structurally defined side-on peroxide complexes. 

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