Alkylmetals electron transfer

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,)...

Figure 16. Relationship between the activation jree energy and the driving force for electron transfer for alkylmetals to TCNE (left) and IrCl6z (right) according to Marcus Equation 4.

Hexachloroiridate ion, IrClJ-, is a complex inert to substitution and is known to undergo outer-sphere electron transfer with other inorganic species (cf. Cecil and Littler, 1968). Some of its reactions have been treated in Tables 12 and 14 and shown to be of the non-bonded electron-transfer type. Its reaction with various alkylmetals has been thoroughly studied, and some results are shown in Table 16 (entries nos. 14 and 15). Except for sterically hindered tetralkyltins the Marcus theory makes incorrect predictions for these reactions, and non-bonded electron transfer does not appear to be feasible. 

Figure 19. Correlation of the activation energy with the free energy (ACet) for electron transfer from alkylmetals (as...

Since the fate of the redox pair must be back electron-transfer (in the absence of a diversionary reaction), interest centers on chemical reactions fast enough to obviate the back electron transfer process in Scheme V. Clearly, instability in the reduced acceptor or oxidized donor can promote efficient photoreaction. For example, oxidized alkylmetal donors are unstable, and the charge-transfer photolysis of R4M (M = Sn and Pb) in molecular complexes with TCNE is a convenient source of alkyl radicals [206] ... 

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