Integration of electron density and molecular orbital techniques to reveal questionable bonds: The test case of the direct Fe-Fe bond in Fe ₂(CO)₉

The article illustrates the advantages of partitioning the total electron density p(r_b), its Laplacian ∇²ρ(r_b), and the energy density H(r_b) in terms of orbital components. By calculating the contributions of the mathematically constructed molecular orbitals to the measurable electron density, it is possible to quantify the bonding or antibonding character of each MO. This strategy is exploited to review the controversial existence of direct Fe-Fe bonding in the triply bridged Fe₂(CO)₉ system. Although the bond is predicted by electron counting rules, the interaction between the two pseudo-octahedral metal centers can be repulsive because of their fully occupied t_2g sets. Moreover, previous atoms in molecules (AIM) studies failed to show a Fe-Fe bond critical point (bcp). The present electron density orbital partitioning (EDOP) analysis shows that one σ bonding combination of the t_2g levels is not totally overcome by the corresponding σ* MO, which is partially delocalized over the bridging carbonyls. This suggests the existence of some, albeit weak, direct Fe-Fe bonding.