The non-classical carbonyl complex [Hg{Fe(CO)5}2]2+ [SbF6]−
2 is prepared by reaction of Hg(SbF6)2 and excess
Fe(CO)5 in anhydrous HF. The single-crystal X-ray structure
reveals a linear Fe–Hg–Fe moiety as well as an eclipsed
conformation of the eight basal CO ligands. Interestingly, the Hg–Fe
bond length of 2.5745(7) Å is relatively similar to the corresponding
Hg–Fe bonds in literature-known [Hg{Fe(CO)4}2]2– dianions (2.52–2.55 Å),
which intrigued us to analyze the bonding situation in both the dications
and dianions with the energy decomposition analysis with natural orbitals
for chemical valence (EDA-NOCV) method. Both species are best described
as Hg(0) compounds, which are also confirmed by the shape of the HOMO-4
and HOMO-5 of the dication and dianion, respectively, in which the
electron pair is located mainly at the Hg. Furthermore, for the dication
and the dianion, the σ back-donation from Hg into the [Fe(CO)5]2
2+ or the [Fe(CO)4]2
2– fragment is the most dominant orbital
interaction and surprisingly these interaction energies are also very
similar even in absolute values. The fact that both iron-based fragments
are missing two electrons explains their prominent σ-acceptor
properties.