It is usually stated that the two iron atoms in Fe2(CO)9 share a direc
t bond. A theoretical analysis, however, shows that this matter is mor
e 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 stru
cture 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 e
xist, which is, however, hidden by a relatively large effective Fe-Fe
repulsion. Consequently, the bioctahedron is linked together through i
ron-bridge bonds.