Electron-transfer Intermediates in Organometallic Reactions

The homolysis of a metal alkyl may be regarded as a reductive elimination however, it is convenient to consider such reactions separately (Section V,B). Electron-transfer mechanisms for organometallic intermediates in catalytic reactions have been reviewed (129) examples are in the formation of transient RCu(I) or RCr(III) in oxidation (by Cu(II)) or reduction (by Cr (II)) of R, and in the role of Fe in the Kharasch-Grignard... 

For a variety of 18 VE carbonylmetal precursors which contain a labile 2 e-donor ligand such as THF, the differences between anodic peak potentials for oxidation and the reduction potential of the strong n acceptor TCNE vary considerably (Table 2). A thus preset electron transfer between the two reaction partners, the organometallic precursor and TCNE, leads to various amounts of substitutionally labile 17 valence electron precursor complexes as essential intermediates in the catalytic chain (13) [22,56]. 

Provided electron transfer between the electrode and solute species is not interrupted by the coating, even electroinactive films can offer interesting applications. Thus, a chiral environment in the surface layer may impose stereoselectivity in the follow-up reactions of organic or organometallic intermediates. Furthermore, polymer layers may be used to obtain diffusional permeation selectivity for certain substrates, or as a preconcentration medium for analyzing low concentration species. 

In the mid-1960s, Dessy and coworkers [12, 13] provided an extensive survey of the anodic and cathodic reactions of transition metal organometallic species, including binary (homoleptic) carbonyls, and this provided a stimulus for many later detailed studies. Whereas the electrochemistry of heteroleptic transition metal carbonyls is covered elsewhere in this volume, that of the binary carbonyls, which is covered here, provides paradigms for the electrochemistry of their substituted counterparts. A key aspect is the generation of reactive 17-electron or 19-electron intermediates that can play key roles in the electrocatalytic processes and electron-transfer catalysis of CO substitution by other ligands. 

Modern electrochemical methods provide the coordination chemist with a powerful means of studying chemical reactions coupled to electron transfer and exploiting such chemistry in electrosynthesis. In addition, the electrochemical generation of reactive metallo intermediates can provide routes for the activation of otherwise inert molecules, as in the reduction of N2 to ammonia,50 and for electrocatalyzing redox reactions, such as the reduction of C02 to formate and oxalate,51 the oxidation of NH3 to N02-,52 and the technologically important oxidation of water to 02 or its converse, the reduction of 02 to water.53 Electrochemical reactions involving coordination compounds and organometallic species have been extensively reviewed.54-60... 

Nickel catalysis is a very active field in organometallic and organic chemistry (selected reviews [3-7]). Complexes of all oxidation states are active in two-electron transfer processes, such as oxidative addition or reductive elimination as well as in single electron transfer initiating radical reactions. Through these processes, oxidation states from Ni(0) to Ni(III) can be easily accessed under mild conditions. Occasionally, Ni(IV) intermediates were also proposed. Apart from the vast number of Ni(II) complexes, a number of organonickel(I) complexes were characterized by X-ray crystallography and their potency as active species in catalytic cycles tested [8-10]. Either radical or two-electron reactivity was observed. Recently, the structure of some alkylnickel(III) complexes was also structurally elucidated [11]. 

Neither the mechanism for all addition reactions of hydride donors to the carbonyl carbon nor the mechanism for all addition reactions of organometallic compounds to the carbonyl carbon is known in detail. It is even doubtful whether only ionic intermediates occur. For instance, for some LiAlH4 additions an electron transfer mecha-... 

Previously the electron transfer reactions attracted more attention of researchers [1011,1012], Electrochemical data mainly in common with ESR spectroscopy data are the important source of the information about the reaction mechanism and also about structure, reactivity, properties of intermediate free radicals of different classes of organic, organometallic, and inorganic reactions. Elucidation of the mechanism and problems of reactivity in the chemistry of one-electron transfer can be of main significance in such fields as synthesis and catalysis, radical chemistry, photochemical synthesis, biochemistry of in vivo organism. 

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