Alkylmetals ionization

In these alkylmetals of the main group elements, ionization occurs from the highest occupied molecular orbital (HOMO) which has a-bonding character, i.e., they are a-donors. Consequently... 

Figure 3. Direct relationship between the ionization potentials of alkyl radicals (R ) with ID of the alkylmetals RHgMe (O) and R2SnMe2 (%),...

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 15. Contrasting behavior of IrCl62 (+) and TCNE (O) relative to Fe(phen)s3 ((3) in the correlation of rates (log kj of oxidation with the ionization potentials of alkylmetals (identified by numbers in Reference 3).

As already mentioned, ionization potentials have often been correlated with electrochemical ones, most often with Evv Here one finds correlations covering both a limited range of compounds, such as aromatic hydrocarbons [(85), Pysh and Yang, 1964 (86), Neikam and Desmond, 1964] bicyclo[1.1.0]butane derivatives [(87), Gassman et al., 1979 ]2, and homoleptic alkylmetals [ (88), Klingler and Kochi, 1980] and a very extended range of different types of aliphatic and aromatic compounds [(89), Miller et al., 1972]. For the E°/IP set... 

Table 1. Vertical ionization potentials IP) and oxidation (peak) potentials ( ox) of alkylmetals...

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. 

The reversibility of the zwitter ion adduct formation in Eq. 8.6 also affected the rate law of the formation of amino-substituted alkylmetal complexes. Thus, kinetic studies indicated [33] that the rate of the formation of /i-aminoalkyl complex 4 in Scheme 8.20 was second-order with respect to the concentration of the amine, namely rate = [amine] [complex]. This is consistent with a reaction sequence shown in Scheme 8.20 involving a reversible formation of the zwitter ionic intermediate, followed by the rate-determining deprotonation by the second amine molecule. The observed rate constant appeared to contain contributions from both the equilibrium constant of the first step and the rate constant of the second deprotonation, so that the direct comparison of the rate of the initial nucleophilic attack at the coordinated alkene between Pd and Pt complexes was not possible. However, the higher overall reactivity (ca. 70 times) of Pd complex than Pt complex was consistent with the higher ionization potential of Pd than Pt. This difference in the ionization potential then would lead to the weaker jt basicity of Pd(II) than Pt(II) for jt back-donation to alkene jt orbital, and therefore facilitated the nucleophilic attack at the Pd-alkene complex more than that at the Pt complex. 

Table II Lowest Vertical Ionization Potentials of Group lib Alkylmetals. ...

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]. 

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