Outer-sphere complex defined

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. 

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)... 

Owing to the tetragonal distortion of the cupric ions (7), this ion pair must be considered an anisotropic complex as defined by J0rgensen 22)— i.e., an intermediate between an inner- and outer-sphere complex. 

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. 

In the first stage, Reaction 12.28, the surface is protonated, and the anion adsorbs in the 1 plane as outer sphere complex, losing two protons the protons cancel out but are retained here to remark the interface planes involved, which define the contributions to the electrostatic work. In Reaction 12.29, the anion binds specifically, and two water molecnles are removed from the 0 plane becanse water has a dipolar moment, there is an electrostatic work involved, which is not considered in the original TLM. The electrostatic factor involved in Reaction 12.28 results from H2ASOJ adsorbing at the 1 plane, two protons adsorbing at the 0 plane, and two other protons desorbing from the 1 plane ... 

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. 

The nature of outer-sphere alkali metal cations can actually define the ionic equilibrium and also has an affect on complex anions. Fig. 77 illustrates the influence of the cationic surrounding on the wave numbers. 

Outer-sphere (OS) reaction rates and rate laws can be defined for solvolysis of a given complex. Complex formation is defined as the reverse reaction—that is, replacement of solvent (S) by another ligand (L )- Following the arguments of... 

Noyori and coworkers reported well-defined ruthenium(II) catalyst systems of the type RuH( 76-arene)(NH2CHPhCHPhNTs) for the asymmetric transfer hydrogenation of ketones and imines [94]. These also act via an outer-sphere hydride transfer mechanism shown in Scheme 3.12. The hydride transfer from ruthenium and proton transfer from the amino group to the C=0 bond of a ketone or C=N bond of an imine produces the alcohol or amine product, respectively. The amido complex that is produced is unreactive to H2 (except at high pressures), but readily reacts with iPrOH or formate to regenerate the hydride catalyst. 

Ion pairs are outer-sphere association complexes, which have to be clearly distinguished from the organometallic complexes discussed in Section 6. Ion pair formation appears to be much less important in biological membranes as compared with octanol, because the charge of the ions at the membrane interphase can be balanced by counter charge in the electrolyte in the adjacent aqueous phase. The reactions involved in ion pair formation are depicted in Figures 5b for acids and 5c for bases, and the equilibrium constant K ix is defined as follows ... 

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... 

Binding of the components in a neutral aqueous solution was confirmed by potentiometric titrations. The feasibility of electron transfer between the components was predicted by cyclic voltammetry and an efficient outer-sphere fast electron transfer was foreseen. Fluorescence spectroscopy measurements showed that the formation of a defined donor-acceptor complex worked even in water at neutral pH. Electron transfer as the quenching mechanism was proved by laser flash photolysis. 

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