IS THERE A DIRECT FE-FE BOND IN FE2(CO)9

It is usually stated that the two iron atoms in Fe2(CO)9 share a direct bond. A theoretical analysis, however, shows that this matter is more complex. By populating or depopulating certain frontier orbitals of Fe2(CO)9 and optimizing the corresponding hypothetical systems one can split up the interactions, the sum of which leads to the overall structure of the Fe2(CO)9 complex. The different interactions between the complex fragments can be connected with certain molecular orbitals and identified with respect to their bonding or antibonding character. It turns out that only a small direct Fe-Fe attractive interaction can exist, which is, however, hidden by a relatively large effective Fe-Fe repulsion. Consequently, the bioctahedron is linked together through iron-bridge bonds.