A NEW INSIGHT FROM QUALITATIVE MO THEORY INTO THE PROBLEM OF THE FE-FE BOND IN FE2(CO)9

A revision of EHMO calculations with FMO analysis has provided new qualitative insight into the bonding network in Fe2(CO)9 and reconciled the previous diverging interpretations. After separation of all of the Fe(CO)bridge bonding/antibonding levels (12 electrons for six FeC bonds) there remains a two-electron/two-orbital interaction (σ/σ{black star}) that is ultimately responsible for a direct (through-space) FeFe bond. However, predominant repulsive interactions via the CO bridges (through-bond) may hide the linkage.