Inner-sphere electron transfer oxidative addition

A related example of inner-sphere reaction is shown in reaction (5), where an additional mechanistic subtlety appears.10 As shown by the products, inner-sphere electron transfer also occurs but now by remote attack in which the sites of bridging ligand binding to the reductant and oxidant are at different atoms on the bridging ligand. 

Ito T, Shinohara H, Hatta H, Nishimoto S-l (1999) Radiation-induced and photosensitized splitting of C5-C5 -linked dihydrothymine dimers product and laser flash photolysis studies on the oxidative splitting mechanism. J Phys Chem A 103 8413-8420 ItoT, Shinohara H, Hatta H, Fujita S-l, Nishimoto S-l (2000) Radiation-induced and photosensitized splitting of C5-C5 -linked dihydrothymine dimers. 2. Conformational effects on the reductive splitting mechanism. J Phys Chem A 104 2886-2893 ItoT, Shinohara H, Hatta H, Nishimoto S-l (2002) Stereoisomeric C5-C5 -linked dehydrothymine dimers produced by radiolytic one-electron reduction of thymine derivatives in anoxic solution structural characteristics in reference to cyclobutane photodimers. J Org Chem 64 5100-5108 Jagannadham V, Steenken S (1984) One-electron reduction of nitrobenzenes by a-hydroxyalkyl radicals via addition/elimination. An example of an organic inner-sphere electron-transfer reaction. J Am Chem Soc 106 6542-6551... 

Prior coordination of Cl to a variety of Pt(II) substrates has also been observed to precede various oxidations that proceed by inner-sphere electron transfer or atom transfer) 193). Nucleophilic solvents might be expected to mimic the role of the anions in some systems, and examples are known. Thus Mel addition to the rho-dium(I) complex of Scheme 20 is in competition with a second reaction that is dependent on solvent addition 194). Rate law (25) was observed, As being the solvent dependent part. 

Scheme 3.20 Oxidative addition via an inner-sphere electron transfer.

Despite intense study of the chemical reactivity of the inorganic NO donor SNP with a number of electrophiles and nucleophiles (in particular thiols), the mechanism of NO release from this drug also remains incompletely understood. In biological systems, both enzymatic and non-enzymatic pathways appear to be involved [28]. Nitric oxide release is thought to be preceded by a one-electron reduction step followed by release of cyanide, and an inner-sphere charge transfer reaction between the ni-trosonium ion (NO+) and the ferrous iron (Fe2+). Upon addition of SNP to tissues, formation of iron nitrosyl complexes, which are in equilibrium with S-nitrosothiols, has been observed. A membrane-bound enzyme may be involved in the generation of NO from SNP in vascular tissue [35], but the exact nature of this reducing activity is unknown. 

In terms of the development of an understanding of the reactivity patterns of inorganic complexes, the two metals which have been pivotal are platinum and cobalt. This importance is to a large part a consequence of each metal having available one or more oxidation states which are kinetically inert. Platinum is a particularly useful element of this pair because it has two kinetically inert sets of complexes (divalent and tetravalent) in addition to the complexes of platinum(O), which is a kinetically labile center. The complexes of divalent and tetravalent platinum show significant differences. Divalent platinum forms four-coordinate planar complexes which have a coordinately unsaturated 16-electron d8 platinum center, whereas tetravalent platinum is an 18-electron d6 center which is coordinately saturated in its usual hexacoordination. In terms of mechanistic interpretation one must therefore consider both associative and dissociative substitution pathways, in addition to mechanisms involving electron transfer or inner-sphere atom transfer redox processes. A number of books and articles have been written about replacement reactions in platinum complexes, and a number of these are summarized in Table 13. 

Electrogenerated monovalent Co complexes of the well-known open chain N202 Schiff base ligands salen (8), salphen (9), and their substituted derivatives undergo oxidative additions with alkyl halides. Reactions of the complex with substrates within the series RBr (R = Pr, Bu, t-Bu) proceed at different rates. The reaction occurs by an inner-sphere alkyl-bridged electron transfer, with a Co1- R+- X-transition state, which is sensitive to distortions of the complex in different configurations.124... 

In addition, the determination of metal-ligand bond distances in solution and their oxidation state dependence is critical to the application of electron transfer theories since such changes can contribute significantly to the energy of activation through the so-called inner-sphere reorganizational energy term. 

PAsl00ph45 refers to 100pM of Zn presorbed prior to the 100 J,M As(III) addition at pH 4.5, and SAslOOphb refers to the simultaneous 100 J,M Zu/lOOpM As(III) addition at pH 6.0. Even though adsorbed Zn was present in the system, As(III) readily oxidized over time. However, Power et al. (2005) suggest that Zn is likely to form inner-sphere complexes on bimessite surfaces and chemisorbed Zn ions inhibit electron-transfer reactions. When Zn was present, As(in) oxidation was further suppressed by nonadsorbed and preadsorbed Zn, compared to the control system, but the preadsorbed system was more effective in interfering with electron-transfer reactions. 

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. 

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