Iron carbonyl complexes are an integral part of organometallic chemistry, existing in a large range of formal oxidation states from -II (in [Fe(CO)4] 2-) to +IV in [Cp*2FeCO] 2+ . However, even the most prominent compound, Fe(CO)5, which is known for more than 130 years, still provides formidable challenges for nowadays inorganic chemists. Being an 18 valence electron complex, Fe(CO)5 is a weak base and protonation is only achieved under superacidic conditions. It was not until 2014, when Krossing et al. succeeded in the first structural characterization of a heterobimetallic complex in which Fe(CO)5 acted as a ligand towards a transition metal cation. Interestingly, [Ag{Fe(CO)5}2] + [Al(OR F )4] -was serendipitously discovered during attempts to oxidize Fe(CO)5 with Ag + [Al(OR F )4] -. The synthesis of the originally desired [Fe(CO)5] + was only achieved in 2022 by a silver-free route. Related heterobimetallic Ag-Fe and Au-Fe complexes containing the Fe(CO)5 moiety were later synthesized in the Dias group. In this context also a combined experimental and theoretical study by Dias and Frenking about the complete series of group 11 complexes [M{Fe(CO)5}2] + (M = Cu, Ag, Au) is especially noteworthy. In comparison, the corresponding group 12 chemistry has received very little attention and no structurally characterized complexes exist. Due to our recent interest in mercury chemistry, we wondered whether Hg(II) compounds could form isolable complexes with Fe(CO)5 as a ligand. Whereas mercury halides or organomercury compounds are only mild Lewis acids, "naked" Hg 2+ ions, generated from HgF2 and the powerful Lewis acids AsF5 or SbF5 are extremely electrophilic, allowing the isolation of complexes with weak Lewis bases, e.g. XeF2 or KrF2 (coordination via F) or even elemental xenon itself! Indeed, the reaction of Hg(SbF6)2 with Fe(CO)5 in anhydrous HF gave the desired heterometallic product. Single crystals of [Hg{Fe(CO)5}2] 2+ [SbF6] - 2 •2HF were obtained upon cooling to -75°C. The compound crystallizes in the monoclinic space group P21/n. Since the asymmetric unit contains only a [HgFe(CO)5] fragment, the Fe-Hg-Fe angle is perfectly linear (180°) with an Fe-Hg distance of 2.5745(7) Å (see Figure ). The iron centers display an octahedral coordination geometry. Regarding the basal carbonyl ligands of both iron centers, an eclipsed conformation is observed. Interestingly, these eight CO ligands are slightly bent towards the metal center, resulting in Hg-Fe-C angles of 83.22° to 86.40° and intramolecular Hg-C contacts between 2.985-3.079 Å, however this distortion is less pronounced than in [Au{Fe(CO)5}2] + (see Table ). Due to their positive charge all carbon atoms show short contacts to two to three negative polarized fluorine atoms of neighboring [SbF6] -anions or HF molecules. The C … F distances are in the range of 2.828-3.161 Å and therefore below the sum of van der Waals radii of C and F (3.17 Å). Scheme 1: Synthesis of [Hg{Fe(CO)5}2] 2+ [SbF6] - Interestingly, the [Hg{Fe(CO)5}2] 2+ dication structurally resembles the literature-known dianion [Hg{Fe(CO)4}2] 2-which can be prepared from K2[Fe(CO)4] and HgCl2 in a 2:1 ratio. Consequently, a comparison of these species seemed worthwhile. Although the reported crystal structures with [Ph3PNPPh3] + and [Na(THF)2] + counter cations are of good quality, the relatively low precision of some bond lengths as well as the cation-anion interactions made it more reasonable to compare the optimized gas phase structures of [Hg{Fe(CO)5}2] 2+ and [Hg{Fe(CO)4}2] 2-. Therefore, we calculated the molecular structures of [Hg{Fe(CO)5}2] 2+ and [Hg{Fe(CO)4}2] 2-using density functional theory at the BP86-D3(BJ)/def2-TZVPP level and we analysed the electronic structure with various methods in order to elucidate the Hg-Fe bonding situation. Table shows that the calculated geometries of the bare ions are in good agreement with X-ray structural data of ionic compounds, considering the influence of solid state forces and the counter ions on the measured values. The calculations suggest that both species have a staggered equilibrium geometry with D4d symmetry for [Hg{Fe(CO)5}2] 2+ and D3d symmetry for [Hg{Fe(CO)4}2] 2-, which are only slightly lower in energy than the eclipsed forms (Table ). The experimental IR spectrum of [Hg{Fe(CO)5}2] 2+ [SbF6] - 2 displays one band of moderate intensity at ṽ = 2156 cm -1 and one intense and broad band at ṽ = 2099 cm -1 . These values are in good agreement with the calculated values (BP86-D3(BJ)/def2-TZVPP) (ṽ = 2152, 2120, 2100 cm -1 ). The predicted values for the Raman-active bands (ṽ = 2171, 2125, 2114 (2x), 2094 (2x) cm -1 ) also nicely fit the experimentally observed ones (ṽ = 2182, 2143, 2137, 2122, 2115 cm -1 ). Slight deviations between the experimental and calculated values might be a consequence of cation-anion interactions in the solid state. Since the highest ṽCO values are clearly above the threshold of ṽ = 2143 cm -1 of free CO, [Hg{Fe(CO)5}2] 2+ can also be classified as a non-classical carbonyl complex. Although a clear blueshift of the CO bands in comparison to Fe(CO)5 is apparent (IR: ṽ = 2020, 1989 cm -1 , Raman: ṽ = 2114, 2028, 1985 cm -1 ), the values are not as extreme as in [Fe(CO)6] 2+ (IR: ṽ = 2204 cm -1 , Raman: ṽ = 2241 and 2220 cm -1 ) or [Hg(CO)2] 2+ (IR: ṽ = 2278 cm -1 , Raman: ṽ = 2281 cm -1 ). In comparison, the IR spectrum of the dianion in [Ph3PNPPh3]2[Hg{Fe(CO)4}2] displays significantly red-shifted bands (ṽ = 1974, 1939, 1862, 1824 cm -1 ) which are a consequence of strong π-backbonding of the electron-rich iron atoms into the π*(CO) orbitals. In order to understand the bonding situation in [Hg{Fe(CO)4}2] 2-and [Hg{Fe(CO)5}2] 2+ we calculated partial charges using different methods (Table ). All methods indicate that the Hg atom carries a positive partial charge in the dication as well as in the dianion. The NBO method even suggests an identical positive charge of q(Hg) = 0.69 e in both species, which is significantly larger than the other methods, and it also indicates a rather large negative partial value for Fe in the dication and in the dianion. This could be due to the fact that the NBO does not consider the unoccupied (n)p-AOs of transition metals (TM) as valence orbitals and that the proponents of the NBO therefore proposed to replace the 18-electron rule for TM compounds with a 12-electron rule. This has been criticized by several workers and it has been shown that NBO gives unreasonable partial charges in carbonyl complexes. We think that the smaller positive charge at Hg given by the other methods is a more plausible description of the charge distribution. Table shows that the EDA-NOCV method suggests a very similar bonding situation in the dication and dianion. The total interaction ∆Eint between Hg atom and the [Fe(CO)n]2 2+