Outer-sphere complex Redox reactions

Only simple outer-sphere (25) redox reactions involving, for example, complex or aquo ions of transition or certain rare earth elements do not experience electrocatalysis, and their standard rate constants are independent of electrode material. This is because neither the oxidized nor the reduced species are chemisorbed at the electrode. However, practically, many redox systems do experience electrocatalysis on account of significant adsorption of their ions or through mediation of electron transfer by adsorbed anions, in which case the processes are no longer strictly of the outer-sphere type. 

Figure 8-42 illustrates the anodic and cathodic polarization curves observed for an outer-sphere electron transfer reaction with a typical thick film on a metallic niobium electrode. The thick film is anodically formed n-type Nb206 with a band gap of 5.3 eV and the redox particles are hydrated ferric/ferrous cyano-complexes. The Tafel constant obtained from the observed polarization curve is a- 0 for the anodic reaction and a" = 1 for the cathodic reaction these values agree with the Tafel constants for redox electron transfers via the conduction band of n-lype semiconductor electrodes already described in Sec. 8.3.2 and shown in Fig. 8-27. 

If the reactants are oppositely charged, the collision complex in (5.18) takes the form of an outer-sphere complex with discernable stability. For the outer sphere redox reaction between Co(NH3)jL"+ and Fe(CN)g, L being a series of pyridine or carboxylate derivatives, saturation kinetics are observed, with the pseudo first-order rate constant (/ obs)> Fe(II) in excess, being given by... 

Tris(l,2-bis(dimethylphosphino)ethane)rhenium(I), [Re(DMPE)3]+ is a simple, symmetrical cation which contains three identical bidentate phosphine ligands. This complex provides a Re(II/I) redox couple with properties that are very convenient for the study of outer-sphere electron transfer reactions.1 Specifically, this couple is stable in both alkaline and acidic media and it exhibits a reversible, one-equivalent redox potential in an accessible region [ °,(II/I) = 565 mV vs. NHE]. Moreover, this complex has been used to obtain information about the biological mechanism of action of 186Re and l88Re radiopharmaceuticals.2,3... 

The stereoselective reduction of spinach plastocyanin with several cobalt cage complexes (Scheme 26) has been reported, too [60]. These cage complexes are very useful for investigation of outer-sphere electron transfer reactions because of their inertness to hydrolysis and to loss of ligands in the redox reaction. 

The biomethylation reaction between platinum and methylcobalamin involves both platinum(II) and platinum(IV) oxidation states. An outer-sphere complex is formed between the charged platinum(II) salts and the corrin macrocycle, which catalytically labilizes the Co—C o bond to electrophilic attack. A two-electron redox switch mechanism has been proposed between platinum(II) and platinum(IV). However, a mechanism consistent with the kinetic data is direct electrophilic attack by PtClg on the Co—C a bond in MeBu. Studies on [Pt(NH3)2(OH2)2] indicate that the bases on cobalt interact in the coordination sphere of platinum(II). Since both platinum(ll) and platinum(rV) are together required to effect methyl transfer from methylcobalamin to platinuni, Pt and C NMR spectroscopy have been used to show that the methyl group is transferred to the platinum of the platinum(n) reactant. The kinetics of demethylation by mixtures of platinum(II) and platinum(IV) complexes show a lack of dependence on the axial ligand. The authors conclude therefore that it is unlikely that the reaction involves direct attack by the bound platinum on the Co—C bond, and instead favor electron transfer from an orbital on the corrin ring to the boimd platinum group in the slow step, followed by rapid methyl transfer. ... 

Type 1. The characteristic chemical properties of these centers are their apparent isolation from the medium and ability to undergo rapid outer sphere electron transfer reactions. The common physical properties which signify this type of Cu complex are an intense absorption envelope centered around 600 nm made up of several absorption bands, and the unusually low hyperfine coupling constant. An. These properties are similar for all known Type 1 centers, and they suggest a common structure. However, Type 1 binding sites show considerable variation in their redox potentials (200—800 mV) and their sensitivity toward denaturation by mercurials. 

In contrast to cerium, outer-sphere electron transfer appears to be the dominant reaction route for europium redox reactions with both organic species and transition metal complexes. Interpretation of europium redox reactions in terms of the Marcus theory of outer-sphere electron transfer reactions is limited by the enduring controversy over the self-exchange rate for the Eu(II)/Eu(III) couple. Since the selfexchange rate for the Eu(II)/Eu(IlI) couple has so far proven inaccessible to direct... 

Reaction 3. This reaction has been described as a "Redox-Switch" mechanism. The metal ion in its lower oxidation state first forms an "outer sphere" complex with the corrin macrocycle, followed by a two electron transfer of the complexed metal ion to a two electron acceptor. Oxidation of the complexed metal ion facilitates carbanion transfer from the cobalt to the complexed metal. 

The simplest type of redox reaction is the outer sphere self exchange reaction which can be represented in terms of contact between the outer electron clouds of the reacting complexes. When the contact occurs the electron jumps from an orbital on the reducing complex to an orbital on the oxidising complex. Fig. 6.1... 

The most important theoretical ideas concerning adiabatic outer sphere electron transfer reactions in solution are summarized. The kinetics of the reduction of a series of different tris-1,10,-phenanthroline complexes of Fe(III) by Fe(CN) were measured in order to test the influence of the redox-potential on these reactions. The lectron exchange rate of the complexes Fe(dipy), Ru(dipy) and 0 (dipy) was derived from the study of their reduction by Fe(CN). Using the edox reaction between the anionic complexes of Fe(CN) and the effect of added... 

The remarkable physical properties exhibited by the divalent macrobicyclic cage complex [Co(sep)]2+ (29) are unparalleled in Co chemistry.219 The complex, characterized structurally, is inert to ligand substitution in its optically pure form and resists racemization in stark contrast to its [Co(en)3]2+ parent. The encapsulating nature of the sep ligand ensures outer sphere electron transfer in all redox reactions. For example, unlike most divalent Co amines, the aerial oxidation of (29) does not involve a peroxo-bound intermediate. 

The aim in solution studies on metalloprotein is to be able to say more about intermolecular electron transfer processes, first of all by studying outer-sphere reactions with simple inorganic complexes as redox partners. With the information (and experience) gained it is then possible to turn to protein-protein reactions, where each reactant has its own complexities... 

In heterogeneous redox reactions similar reaction sequences are observed usually an encounter (outer-sphere or inner-sphere) surface complex is formed to facilitate the subsequent electron transfer. 

In addition to simple reactions of electron transfer (outer-sphere electron transfer) between an electrode and hydrated redox particles, there are more complicated reactions of electron transfer in which complexation or adsorption of redox particles is involved. In such transfer reactions of redox electrons, the redox particles are coordinated with ligands in aqueous solution or contact-adsorbed on the electrode interface before the transfer of their redox electrons occurs after the transfer of electrons, the particles are de-coordinated from ligands or desorbed from the electrode interface. 

Undoubtedly one of the most used LFER in transition metal chemistry involves electron transfer rate constants and associated equilibrium constants in outer sphere redox reactions. These are an unusual class of reactions in chemistry since bonds are only stretched or contracted in the formation of the activated complex. They therefore lend themselves well to theoretical treatment. We shall have more to say about these reactions in Chap. 5. It is sufficient here to state the simple form of the LFER with an example (Fig. 2.8). For the reaction... 

Negative or very small values of 7s.H are rare. They obviously cannot arise from a single step, and they give overwhelming evidence for a multistep process that includes a preequilibrium. Negative or near-zero values for AFT for a few inner-sphere and outer-sphere redox reactions indicate the occurrence of intermediates, and rule out a single step, with a single activated complex (Sec. 5.5). 

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