Ionization potential metallocenes

The appearance potentials for molecular ions (ionization potentials) and for fragment ions formed in the mass spectra of metallocenes and related compounds are listed in Table XIII. These appearance potentials have been used to calculate bond dissociation energies and heats of formation of organometallic compounds, but the results obtained must be treated cautiously because the appearance potentials of fragment ions include excess energy due to excited species. The values obtained for the heats of formation are best considered as upper limits, rather than precise determinations. The extent to which energy due to excited states can contribute... 

Comparison of Ionization Potentials of Metallocenes with Ionization Potentials of the Free Metal Atoms... 

The importance of solvent effects can be extracted from correlations of half-wave potentials determined by CV or polarographic methods with other measures of oxidizability, especially gas-phase free energies of ionization and ionization potentials. Within a series of unsubstituted metallocenes that differ only by the metal, the oxidation potentials of the compounds in solution and their gas-phase free energies of ionization vary directly (Fig. 17),154, 60 indicating that the effect of solvent is consistent from one metallocene to another. 

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. 

Table 2. Vertical ionization potentials [IP] and oxidation potentials ( °ox) of metallocenes and (arene) metal sandwich complexes.

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]. 

Table III Ionization Potentials and Oxidation Potentials of Metallocenes and Other Sandwich Donors.

In substituted metallocenes, however, oxidation potentials do not always correlate well with gas-phase ionization energies. For example, although there is a positive trend between the irreversible oxidation potentials of a series of substituted ruthenocenes and their gas-phase energies of ionization, the quantitative correlation between the two is poor (Fig. 18).161 The weak correlation is believed to reflect changes in solvation that accompany the addition of substituents. 

Fig. 17. Correlation of the gas-phase free energies of ionization and oxidation potentials for unsubstituted metallocenes. The vlP for all compounds are from Ref. 154 the sources of the Ex and values are as follows [CoCp2] and [CrCp J. Refs. 198. 199 [NiCps] and [FeCpi]. Refs. 198. 200 [OsCp2l. Refs. 201. 202.

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