The E1u2 ←E2g2 (16 200 cm−1) band system for the three ferricenium salts [Fe(C5H5)2]PF6, [Fe(C5H5)2]BF4, and [Fe(C5H5)2](CCl3CO2H)2–(CCl3CO2−) has been studied at 4.2°K. Analysis of the observed vibrational structure indicates that the 2E1u excited state is split into two Kramers doublets, with the extent of splitting being a function of the anion. Several ferricenium 2E1u vibrational frequencies have been identified and compared with corresponding values for ground state ferrocene. It appears from these comparisons that the iron 4px and 4py orbitals are minimally involved in the iron-ring bonding.
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Hendrickson et al. (1973) studied this question.