Ionization potential alkylmetal donors

For a particular iron(III) oxidant, the rate constant (log kpe) for electron transfer is strongly correlated with the ionization potential Ip of the various alkylmetal donors in Figure 4 (left) (6). The same correlation extends to the oxidation of alkyl radicals, as shown in Figure 4 (right) (2). [The cause of the bend (curvature) in the correlation is described in a subsequent section.] Similarly, for a particular alkylmetal donor, the rate constant (log kpe) for electron transfer in eq 1 varies linearly with the standard reduction potentials E° of the series of iron(III) complexes FeL33+, with L = substituted phenanthroline ligands (6). 

Figure 4. Correlation of the ionization potentials of alkylmetal donors with the electron-transfer rate constant (log kFe) for Fe(phen)s3+ (%), Fe(bpy)s3+ (O), and Fe(Cl-phen)s3+ ((D), (left). The figure on the right is the same as the left figure for Fe(phen)s3+ except for the inclusion of electron-transfer rates for some alkyl radicals as identified, (Note the expanded scale,)...

Metallocenes are useful electron donors as judged by their low (vertical) ionization potentials in the gas phase and oxidation potentials in solution (see Table 2). In fact, the electron-rich (19 e ) cobaltocene with an oxidation potential of E°ox = -0.9 V relative to the SCE [45] is commonly employed as a very powerful reducing agent in solution. Unlike the alkylmetals (vide supra), the HOMOs of metallocenes reside at the metal center [46] which accounts for two effects (i) Removal of an electron from the HOMO requires minimal reorganization energy which explains the facile oxidative conversion from metallocene to metallocenium. (ii) The metal-carbon bonding orbitals are little affected by the redox process, and thus the resulting metallocenium ions are very stable and can be isolated as salts. 

Ferrocene is a viable electron donor by virtue of its vertical ionization potential of only 6.86 eV in the gas phase [37] and its oxidation potential of only + 0.41 vs SCE [39] in CH3CN solution. Unlike the alkylmetal donors, the one-electron oxidation product, ferricenium cation, is stable, and various salts of it can be isolated. This stability arises from the metal-centered nature of the HOMO (e2g in 05 symmetry) [40] which minimizes the effect of electron removal from the metal-carbon bonding orbital. Indeed, ferrocene and related metallocenes undergo multiple redox reactions without disruption of the sandwich structure [41]. 

---