Outer-sphere competition, inner

Complexation of Pu is discussed in terms of the relative stabilities of different oxidation states and the "effective" ionic charge of PuG" and Pu02 . An equation is proposed for calculating stability constants of Pu complexes and its correlation with experimental values demonstrated. The competition between inner outer sphere complexation as affected by the oxidation state of Pu and the pKa of the ligand is reviewed. Two examples of uses of specific complexing agents for Pu indicate a useful direction for future studies. 

Figure 1. Scheme III reactions in competition with outer-sphere processes. The dotted line represents the outer-sphere path, the solid the inner-sphere path and P and S stand for the precursor and successor complexes, respectively. (A) Successor complex forms rapidly but decays to final products by an outer-sphere mechanism. (B) The entire mechanism is inner sphere.

Trivalent lanthanide and actinide cations form labile, ionic complexes of both inner and outer sphere character. Consequently, they are useful probes to study inner-outer sphere complexation competition due to ligand properties. Two earlier papers have reported complexation of these cations by two series of related anions, the halates (9) and the chloroacetates (10). In this paper we offer a more extensive analysis of the inner-outer sphere competition in these complexes. 

J/m/K. The agreement between the nmr estimates and those from equation (1) add weight to the estimates in Table III. In Figure 2 the variation of log i and log 0 as functions of pKa reflect the vital role of ligand basicity in the inner-outer sphere competition. These curves indicate that the cross-over from predominantly outer sphere to predominantly inner sphere occurs near pKa values of 2. However, since the enthalpy and entropy changes for inner sphere complexation are larger than for outer sphere formation, both AH and AS would still be endothermic (characteristic of inner sphere reaction). 

C3.2.2.12 COMPETITION BETWEEN INNER SPHERE AND OUTER SPHERE NUCLEAR POLARIZATION DYNAMICS... 

The outer-sphere rate constant for the Cr(H20)5+/IrClg reaction can be estimated, using /larcus equation, as =10 M s". A value of this magnitude can obviously be competitive dtb that for the inner-snhere nath. which is more usual with the hiehlv labile CrtH,0)i ... 

It has been shown that a complete shift in stereochemistry of the nucleophilic reactions of (29), with alkyl halides such as 2-bromobutane or cis-2-bromomethoxycyclohexane, from racemization to complete inversion, is induced by increase in the inner-sphere stabilization of the transition state from 0 to 3 kcal mol" This has been ascribed to competition between inner-sphere 5)vr2 and outer-sphere electron-transfer processes the former being extremely sensitive towards inner-sphere stabilization. 

Thus, in hydrogen-transfer reactions, most of the catalysts do prefer the outer-sphere mechanism instead of the MPV or the insertion mechanisms. For instance, the high stability of the intermediate formed, alkoxide in the case of carbonyl hydrogenation, is a major drawback for the inner-sphere mechanism. Nevertheless, in some particular cases, the inner-sphere mechanism may be competitive with the outer-sphere one. In these cases, some requirements must be accomplished, such as the high lability of one of the metal ligands in order to allow easily the substrate coordination or the formation of not very stable intermediates. 

This chapter mainly focuses on the reactivity of 02 and its partially reduced forms. Over the past 5 years, oxygen isotope fractionation has been applied to a number of mechanistic problems. The experimental and computational methods developed to examine the relevant oxidation/reduction reactions are initially discussed. The use of oxygen equilibrium isotope effects as structural probes of transition metal 02 adducts will then be presented followed by a discussion of density function theory (DFT) calculations, which have been vital to their interpretation. Following this, studies of kinetic isotope effects upon defined outer-sphere and inner-sphere reactions will be described in the context of an electron transfer theory framework. The final sections will concentrate on implications for the reaction mechanisms of metalloenzymes that react with 02, 02 -, and H202 in order to illustrate the generality of the competitive isotope fractionation method. 

Inner-sphere complexes are relatively stable in comparison to outer-sphere complexes under equivalent solution conditions (i.e. pH, ionic strength), and in a competitive situation will tend to displace less stable adsorbates. This is a fundamental property of coordination reactions, and explains the observed trends in metal uptake preference observed in lichen studies (Puckett et al., 1973). Metal sorption results previously attributed to ion exchange reactions are more precisely described as resulting from competitive surface complexation reactions involving multiple cation types. Strictly speaking, each metal adsorption reaction can be described using a discrete mass law relation, such as... 

Nevertheless, the analysis given in Table 7 is not highly sensitive to this variable. The same trend is seen in the enhancement by the inner-sphere path, where the inner-sphere/outer-sphere competition is in the tj -tj pair. There are also limited data on the reduction of [FeCl], but this reagent reacts so rapidly that the full enhancement possible cannot be appreciated (see Table 8). What is of interest here is that again the 2g 2g shows little rate enhancement in fact, the inner- and outer-sphere paths... 

If a strongly adsorbing bivalent metal ion is added to the system described by Eqs. (39) and (40), in which competitive adsorption of protons and ions of basic electrolyte occurs, then according to the triple layer model [103-105] its addition can cause the formation of two kinds of surface complexes inner-sphere complexes SOM formed at the 0-plain of the triple layer and outer-sphere complexes SO M + formed at the, 3-plain. Some recent studies by Hayes and Leckie [142-145] suggest that the formation of the inner-sphere complexes is more probable for divalent cations like Cu, Pb, Cd" ", etc. than the formation of outer-sphere surface complexes. So, in general [142,143] ... 

Although the concept of outer sphere complexation was introduced by Werner (1) in 1913 and the theory first given a mathematical base by Bjerrum (2) in 1926, progress in understanding the factors involved in the competition between inner and outer sphere complexation has been very slow. 

In an effort to distinguish between outer-sphere and inner-sphere mechanisms for reductions by 2-tol, the reduction of imines was carried out in the presence of amines as potential trapping agents. The outer-sphere mechanism suggested that unsaturated intermediate A might be competitively trapped by either the newly reduced amine or by the added amine in contrast, the inner-sphere mechanism predicts exclusive formation of the complex of the newly reduced amine (Scheme 13). 

The iron(ii)-dependent retardation has been confirmed, although at high concentrations of Fe it would appear that the rate law may be more complex than that stated, possibly involving a term in iodide. An outer-sphere associated species is preferred to the complex Fel +, and it may be that the ion pair is a precursor to a species of the type [I, Fe +, I ], which imdergoes a relatively rapid electron-transfer reaction in competition with the formation of the inner-sphere complex ... 

The physical sense of molecular structure is not fully tmderstood. But studies of water solutions by various methods indicate that the area of its distribution oversteps botmdaries of the hydrates. In this connection in the molecular structure arotmd each ion are identified two spheres of the molecular structure inner, or internal hydration shell (Figure 1.2), and outer, or external hydration shell (Figure 1.2). Inner or primary hydration shell is positioned within the hydrate and is caused by direct orientation interaction of H O with ions. Outer hydration shell is caused by the competitive efiect on H O from the ion, on the one hand, and from interdipole hydrogen bonds on the other. Such disrupted H O dipole structure is sometimes called cybotactic state. 

For a smectic-A that is confined in the SEA crossed cylinder geometry, there is a competition between the homeotropic alignment on mica and the tendency to form layers of equal thickness d. As a result, dislocations must arise (Fig. 3.18, inset). As the local geometry aroimd the contact point is equivalent to a sphere-plane geometry, the loops are expected to be circular and centered on the contact point. Consider thus an array of torus-like cells, coaxial to the loops (Fig. 3.18, inset). Each cell is defined by an inner radius r, corresponding to a thickness h ri) = Uid, and an outer radius rj+i, with h ri+i) = (rij - - l)d, and contains a circular dislocation loop of radius pi- The cells are independent, because the strain patterns produced by the dislocations decays exponentially outside a parabola of equation = z, where A ... 

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