Structural Characterization and Bonding Analysis of [Hg{Fe(CO)₅}₂]²⁺ [SbF₆]⁻₂

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]₂²⁺ 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.