The d-electrons of Fe in ferrocene: the excess orbital energy spectrum (EOES)

The d-electrons of transition metal Fe play a significant role in characterising properties of the ferrocene (Fc) eclipsed (E) and staggered (S) conformers and their chemical bonding, which must be appropriately included in the basis set for Fe, in addition to the level of theory. The present density functional theory (DFT) based B3LYP/m6-31G(d) model has successfully calculated accurate infrared (IR) spectra of ferrocene without scaling (Mohammadi et al., 2012). The present study introduces an excess orbital energy spectrum (EOES) in order to assess more detailed orbital based information of ferrocene between the conformers, which is contained in this model. It is revealed that the d-electrons of Fe show significantly larger electron correlation energies than other electrons of Fc, which contain the key information for the conformers, and not all the d-electrons of Fe play the same role in the conformer pair of Fc. Inclusion of electron correlation energy in the model, theory as well as the basis set, therefore, becomes critically important to produce reliable and useful results for Fc. It further reveals that the Fe-dominant orbitals, i.e., 4a(1)' (i.e. MO 16), 3e(1)' (MO 18-19), 3e(2)', 3e(2)'' (MO 37-40) and 4e(2)' (HOMO, i.e., MO 47-48), which are all dominated by d-electron contributions from Fe, exhibit excess orbital energies larger than their total electronic energy difference between the conformers. Finally, the present study suggests that the covalent bonding character exists in the Fe-Cp bonds of Fc, which may help explain the reason of high thermal stabilities of Fc.