Theoretical Study of Bonding, Structure, and Vibrational Spectra of the [Fe₂(CO)₈]^2- Anion and Its Derivatives^†

A theoretical study of the structural versatility of the di-iron carbonylate, [Fe₂(CO)₈]^2-, and of its adducts with electrophiles is presented. The geometries of three energy minima and four transition states of [Fe₂(CO)₈]^2- have been characterized, and the relative energies of several alternative structures have been evaluated. The calculated vibrational spectra in the Fe−Fe and the C⋮O stretching regions are discussed for the three isomers, and a good correlation between the Fe−Fe stretching force constant and the Fe−Fe bond distance is found for both theoretical and experimental data. The effect of the orientation of the terminal ligands on the Fe−Fe bond and the rearrangement of such ligands by the formation of adducts with electrophiles are also addressed.