Centered Organometallic Radicals

Reviews Relevant to Organometallic Free Radical Chemistry 

Section of this review Subject/title References 

II/III Free radical rearrangements, including Si2Me6 — Me3SiSiMe2CII2 Me3SiCIT2SiMe2 191 

II/III Structure of free radicals by ESR spectroscopy, including spin trapping (no direct organometallic content) 71 

II/III/IV Organometallic radicals, principally of main group elements, literature survey 1971 to mid-1972 102 

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Structural applications range from organic, inorganic and organometallic radicals to coordination complexes and biological macromolecules containing a paramagnetic center. 

Electron transfer, in thermal and photochemical activation of electron donor-acceptor complexes in organic and organometallic reactions, 29, 185 Electron transfer, long range and orbital interactions, 38, 1 Electron transfer reactions within s- and p-bridged nitrogen-centered intervalence radical ions, 41, 185... 

This chapter will concentrate on the chemistry of metal-14-centered anions (Ge, Sn, Pb). These compounds and their silyl analogues are ionic or polarized alkaline and alkaline earth metal-14 compounds, as well as delocalized molecules such as metalloles. Ammonium metallates Mi4 R4N+ or metal-14-centered anion radicals are also considered. The subject was explored during the 1960s and 1970s and thoroughly reviewed in 1982 and 1995 in Comprehensive Organometallic Chemistry, Vols. I and and for silicon species in a previous volume of this series . By that time the main routes to metal-14 anions were known. Since then, the subject has been developed in the topics of particular syntheses, stabilization using steric hindrance, electronic effects and complexation, spectroscopic and structural analyses "... 

The association and dissociation of ligands at metal centers is a key reaction type in organometallic chemistry, including organometallic radical chemistry. Typically,... 

The ligand-centered reactivity of organometallic radicals has been previously reviewed, and will not be discussed extensively. Only several examples will be shown to illustrate the typical reactivity of these systems will be commented. 

Fragmentation of the C-M bonds of Group 4A organometallic radical-cations usually generates carbon-centered radicals which add to the reduced form of the acceptor to give coupling products [21] (Scheme 11). 

The most synthetic polymers are prepared by a free radical chain-growth polymerization (active centers are radicals). Other active centers are carbocations (cationic chain-growth polymerization, e.g. isobutylene, vinylether, tetrahydrofuran) and carboanions (anioific chain growth polymerization, e.g. butadiene, ethylenoxide, caprolactam). Also organometallic complexes are used as chain carriers (coordination polymerization, e.g. ethylene, propylene, butadiene). [08IUP2]... 

Some of these organometallic radicals are sufficiently stable to be isolable - in particular, many stable 17-electron complexes are known. As organic radicals, organometallic radicals are indeed stabilized by bulk around the metal center that inhibits radical reactions. When they are not sterically stabilized, their main property, beside the redox ones, is the very fast intra- or intermolecular interconversion between the 17-electron and 19-electron forms. The combination of a 17-electron complex with a 2-electron L ligand to give a 19-electron species is schematized below ... 

The first tin-centered radical has been isolated, though it is not strictly organometallic in the C-Sn bonded sense. It is prepared by the reaction shown in Equation (173), in which the SnCl2 acts both as a substrate for forming the stannyl anion and as a one-electron oxidizing agent. 

Electrophilic substitution of thianthrene takes place at C-2. No examples of even minor amounts of 1-mono-substituted product have been reported. Disubstitution gives 2,7- (usually) or 2,8-products. In a few cases, 2,6-derivatives have been claimed. The presence of a sulfoxide or sulfone unit greatly reduces the susceptibility of either ring to electrophilic substitution. Carbon-centered electrophilic addition to sulfur to produce 5-R-thianthrenium salts has been described rarely most examples of the formation of such salts have involved the thianthrene radical ion(l-t-). Treatment of thianthrene with alkyl/aryllithiums produces the 1-lithio-species, and these organometallic derivatives allow the introduction of substituents at this position. 

Secondary inorganic radicals are particularly useful for studying redox changes in metalloproteins and organometallic complexes because these radicals are more likely to react at the metal center by electron transfer, whereas OH will also attack the organic moiety and, by abstracting H, create a reducing radical there. 

The first organometallic compound of the transition metals to be characterized (1827) was Zeise s salt, K[(C2H4)PtCl3]-H20 (Fig. 18.1). It forms when K2[PtCl4] in aqueous ethanol is exposed to ethylene (ethene) a dimeric Pt—C2H4 complex with Cl bridges is also formed. In both species, the ethylene is bonded sideways to the platinum(II) center so that the two carbon atoms are equidistant from the metal. This is called the dihapto-or T]2 mode. A ligand such as an allyl radical with three adjacent carbons directly bonded to a metal atom would be trihapto- or t 3, and so on. 

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