Precursor complex inner-sphere electron transfer

The well-known redox chemistry of provides important insights into the mechanism of the VOHPO4 0.5H2O precursor formation in organic medium. Waters and Littler [24] have shown that most V reductions proceed via a free-radical mechanism where complexation of V to alcohol precedes the one-electron transfer step, i.e. an inner sphere electron transfer. Waters and Littler [24] proposed ternary tetrahedral complex formation between V02, H3O+ and R2CHOH to yield [V(0H)30HCHR2] and observed the following kinetic expression... 

In 1954, King and Taube published the 1980 Nobel Prize winning work that defined these two different types of electron transfer reactions. In an inner-sphere mechanism, the atoms undergoing redox form bonds to a common atom (or small group of atoms), which then serves as a bridge for electron transfer (ISPC = inner-sphere precursor complex and ket = electron transfer rate constant). 

It is important to note for the following discussion that in electron-transfer processes the reductant s highest occupied molecular orbital (HOMO) should combine with the oxidant s lowest unoccupied molecular orbital (LUMO) of the same symmetry to ensure proper overlap of reductant and oxidant orbitals to initiate electron transfer. That is, electron transfer will occur readily from n to n orbitals on different species or from a to a but not n to a in a linear arrangement of atoms [e.g., A-B-C in Appendix I (following references at the end of this chapter)]. In the case of outer-sphere electron-transfer processes, n- to 7r-electron transfers are favored over a to a because (1) such transfers do not require major changes in bond lengths in the precursor complex (lower activation energy) and (2) the n orbitals are more diffuse or better exposed than a orbitals. This process is well documented for transition metals. For inner-sphere electron-transfer processes, both n- to n- and a- to n-electron transfers are most favored (Purcell and Kotz, 1980). 

An inner-sphere mechanism consists of two processes, precursor complex formation and electron transfer, as shown in the following reaction ... 

The Marcus theory model is derived for unimolecular electron transfer. It is applied to bimolecular reactions by assuming that the reactants weakly associate in a precursor complex within which ET occurs to give the successor complex. The cross relation analyses above have implicitly adopted this same model, but HAT precursor complexes are quite different then ET ones. This is because proton transfer occurs only over very short distances, so HAT precursor complexes have distinct conformations, rather than the weakly interacting encounter complexes of ET. In this way, HAT resembles proton transfer and inner-sphere electron transfer. Including the equilibria for precursor and successor complex formation expands equation (1.1) into equation (1.20). 

In contrast, similar plots of AG against AGet for the oxidants TCNE and hexa-chloroiridate(IV) deviate substantially from the simulated curve for outer-sphere electron transfer (see Figures 19B, C). Moreover, the most pronounced deviations are observed with the least hindered tetraalkyltin donors. The fact that steric effects are only observed in the latter cases, but not with the FeL3 acceptor, leads to the conclusion that the inner coordination spheres of the tetraalkyltin donors are perturbed by TCNE and hexachloroiridate in the ET transition state. In other words, the electron transfers to TCNE and iridate(IV) exhibit strong inner-sphere character and thus occur from wai i-five-coordinate precursor complexes reminiscent of a variety of trigonal-bipyramidal structures known for tin(IV) derivatives, i.e. 

Under favorable conditions, then, the three possible transition states for net inner sphere can differ in composition for the usual interchange substitution mechanism there are (1) precursor formation, [ALXB] (2) electron transfer, [AXB] and (3) successor breakdown, [AXL B]. However, L and L usually are solvent, and the number of solvent molecules in an activated complex cannot be determined kinetically. Therefore, under this normal circumstance all three possible transition states have the same composition and cannot be distinguished by direct kinetic measurements only indirect arguments can be used to determine which of the three possible transition states is operative. 

Inner-sphere. Here, the two reactants first form a bridged complex (precursor)- intramolecular electron transfer then yields the successor which in turn dissociates to give the products. The first demonstration of this was provided by H. Taube. He examined the oxidation of ICrfHoOijj by lCoCl(NHr)< and postulated that it occurs as follows ... 

Similarly, inner-sphere and outer-sphere mechanisms can be postulated for the reductive dissolution of metal oxide surface sites, as shown in Figure 2. Precursor complex formation, electron transfer, and breakdown of the successor complex can still be distinguished. The surface chemical reaction is unique, however, in that participating metal centers are bound within an oxide/hydroxide... 

The most direct evidence for surface precursor complex formation prior to electron transfer comes from a study of photoreduc-tive dissolution of iron oxide particles by citrate (37). Citrate adsorbs to iron oxide surface sites under dark conditions, but reduces surface sites at an appreciable rate only under illumination. Thus, citrate surface coverage can be measured in the dark, then correlated with rates of reductive dissolution under illumination. Results show that initial dissolution rates are directly related to the amount of surface bound citrate (37). Adsorption of calcium and phosphate has been found to inhibit reductive dissolution of manganese oxide by hydroquinone (33). The most likely explanation is that adsorbed calcium or phosphate molecules block inner-sphere complex formation between metal oxide surface sites and hydroquinone. 

In the classical formalism it is assumed that bimolecular electron transfer occurs in a precursor complex in which the inner-coordination shells of the reactants are in contact, that is, r - , where a2 and a3 are the hard-sphere radii of the reactants (16). Under these conditions the... 

For an inner-sphere reaction there are necessarily more steps since both association and substitution must precede electron transfer. Intermediates like (H20)5CruClCoUI(NH3)54+ and (H20)5CrinClCoII(NH3)54 shown in Scheme 2 are often referred to as the precursor and successor complexes since they precede or follow the electron transfer step. 

The rate-controlling step in reductive dissolution of oxides is surface chemical reaction control. The dissolution process involves a series of ligand-substitution and electron-transfer reactions. Two general mechanisms for electron transfer between metal ion complexes and organic compounds have been proposed (Stone, 1986) inner-sphere and outer-sphere. Both mechanisms involve the formation of a precursor complex, electron transfer with the complex, and subsequent breakdown of the successor complex (Stone, 1986). In the inner-sphere mechanism, the reductant... 

The first step involves the formation of the precursor complex, where the reactants maintain their identity. In the second step there is, as we will see later, reorganization of the inner coordination shells as well as of the solvation spheres of the reactants so as to obtain a nuclear configuration appropriate to the activated complex through which the precursor complex is transformed into the successor complex. The electron transfer usually occurs during the latter stages of this reorganization process. The activated complex deactivates to form the successor complex if electron transfer has occurred or to reform the precursor complex if electron transfer has not occurred. The electron distribution in the successor complex corresponds to that of the products, so that the third step is simply the dissociation of the successor complex to form the separated products. 

In addition to providing an orbital route for electron transfer, the formation of a stable bridge species is analogous to the formation of the precursor complex required during an inner sphere process12). The overall result is that for a bridged complex, both metals can participate in electron transfer with an external reactant with no additional barriers due to intradimer electron transfer. The advantages for two electron oxidation-reduction reactions are dear. 

Electron transfer from the chromium(II) center to the cobalt(III) center within this inner-sphere precursor complex results in a labile cobalt(II) center and inert chromium(III) center. Concurrent with electron transfer, atom transfer (Cl-) also takes place as Cl-remains bound to the inert chromium(III) center to yield [ClCr(OH2)5]2+ and [(H3N)5Co]2+ decomposes to [Co(H20)6]2+. In 1961, Norman Sutin was able to provide direct mechanistic proof of atom transfer in the inner-sphere process by his introduction of the stopped-flow (rapid mixing) techniqe. The transition state in an inner-sphere process is relatively well defined, and because the kinetics are complex, the overall mechanism is also well defined. 

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