It is somewhat surprising that the dominant orbital interaction in [Hg{Fe(CO)4}2] 2-comes from the Hg(s)[Fe(CO)4]2 2--backdonation, which indicates that the dianion [Fe(CO)4]2 2-is, in spite of its negative charge, an electron acceptor. This is a manifestation of the strong driving force of atoms and fragments to fill their valence shell, which can be stronger than the electrostatic force. [Fe(CO)4]2 2-is short of two electrons to have a filled valence shell, which is partially occupied by the stabilizing Hg(s)[Fe(CO)4]2 2--backdonation that builds the HOMO-5 of the complex (Figure ). Another surprising result concerns the formal oxidation state of Hg in the charged complexes. The EDA-NOCV results suggest that the oxidation state of Hg in [Hg{Fe(CO)5}2] 2+ and [Hg{Fe(CO)4}2] 2-is Hg(0). The same conclusion follows from the assignment of the oxidation state (OS) according to the IUPAC proposal that the OS of an atom is the charge of that atom after ionic approach of its heteronuclear bonds. The shape of the HOMO-4 of the dication and HOMO-5 of the dianion indicates that the electron pair is located mainly at the Hg, making neutral Hg(0) the most correct fragment.