Metallocomplex Anion-Radicals

For instance, the (pentaphenylcyclopentadienyl) cobalt dicarbonyl anion-radical complex [(q-C5Ph5)Co(CO)2] has (n + 1) metal orbital populated with an unpaired electron, according to calculations by Connelly et al. (1986). In contrast, reduction of (bpy)Cr(CO)4 (bpy = 2,2 -bipyridyl) to its anion radical is known to occur without any major change in its structure or composition. 

The ESR spectrum of [(bpy)Cr(C0)4] ion-radical resembles that of the uncoordinated (bpy) anion-radical, indicating clearly that the unpaired electron is localized on the bpy ligand. This conclusion is completely corroborated by the similarity between the electronic spectra of [(bpy)Cr(CO)4] and (bpy) (Vlcek et al. 1998, and references therein). Obviously, the product of one-electron reduction of (bpy)Cr(CO)4 may best be formulated as a formally Cr(0) complex with an anion-radical ligand, (bpy) Cr(CO)4. 

One-electron reduction of metalloorganic complexes or coordination between a metal and an anion-radical ligand may expand an electron shell of the central metal atom. Sometimes, anion-radical metallocomplexes contrast in this regard with the cation-radical ones. Thus, the same metal-loporphyrins form cation-radicals with charges and unpaired electrons on ligands (Shinomura et al. 1981) and anion-radicals with charges and unpaired electrons on metals (Lexa et al. 1989). 

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Alike metallocomplex anion-radicals, cation-radicals of odd-electron structure exhibit enforced reactivity. Thus, the 17-electron cyclopentadienyl dicarbonyl cobalt cation-radical [CoCp(CO)2] undergoes an unusual organometallic chemical reaction with the neutral parent complex. The reaction leads to [Co2Cp2(CO)4]. This dimeric cation-radical contains a metal-metal bond unsupported by bridging ligands. The Co—Co bond happens to be robust and persists in all further transformations of the binuclear cation-radical (Nafady et al. 2006). 

The difference between the two reactions of Scheme 2.9 may also be considered in terms of the complete electron transfer in both cases. If the a-nitrostilbene anion-radical and metallocomplex cation-radical are formed as short-lived intermediates, then the dimerization of the former becomes doubtful. The dimerization under electrochemical conditions may be a result of increased concentration of reactive anion-radicals near the electrode. This concentration is simply much higher in the electrochemical reaction because all of the stuff is being formed at the electrode, and therefore, there is more dimerization. Such a difference between electrode and chemical reactions should be kept in mind. In special experiments, only 2% of the anion-radical of a-nitrostilbene were prepared after interruption of controlled-potential electrolysis at a platinum gauze electrode. The kept potential was just past the cathodic peak. The electrolysis was performed in the well-stirred solution of trani -a-nitrostilbene in AN. Both processes developed in this case, namely, trans-to-cis conversion and dimerization (Kraiya et al. 2004). The partial electrolysis of a-nitrostilbene resulted in redox-catalyzed equilibration of the neutral isomers. 

Scheme 6.27 considers other, formally confined, conformers of cycloocta-l,3,5,7-tetraene (COT) in complexes with metals. In the following text, M(l,5-COT) and M(l,3-COT) stand for the tube and chair structures, respectively. M(l,5-COT) is favored in neutral (18-electron) complexes with nickel, palladium, cobalt, or rhodium. One-electron reduction transforms these complexes into 19-electron forms, which we can identify as anion-radicals of metallocomplexes. Notably, the anion-radicals of the nickel and palladium complexes retain their M(l,5-COT) geometry in both the 18- and 19-electron forms. When the metal is cobalt or rhodium, transition in the 19-electron form causes quick conversion of M(l,5-COT) into M(l,3-COT) form (Shaw et al. 2004, reference therein). This difference should be connected with the manner of spin-charge distribution. The nickel and palladium complexes are essentially metal-based anion-radicals. In contrast, the SOMO is highly delocalized in the anion-radicals of cobalt and rhodium complexes, with at least half of the orbital residing in the COT ring. For this reason, cyclooctateraene flattens for a while and then acquires the conformation that is more favorable for the spatial structure of the whole complex, namely, M(l,3-COT) (see Schemes 6.1 and 6.27). 

There are anion radical metallocomplexes with complete spin retention at the anion radical ligand (Glockle et al. 2001), as well as those having an unpaired electron on a metal atom entirely, or those that share an unpaired electron with all parts of the complex (Kaim 1987 and references therein). 

There is some lack in data on anion radical stabilization as a result of binding into a metallocomplex. One specific case was described for bis(trimethylsilyl) diacetylene (Kaim 1988). When treated with potassium, this compound gives tetrakis(trimethylsily)butate-traene, probably owing to some transformations of the unstable anion radical. This anion... 

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