Mercury(IV) fluoride

  Mercury(IV) fluoride, HgF₄, is the first mercury compound to be discovered with the metal in the oxidation state IV. Mercury, like the other group 12 elements (cadmium and zinc), has an s²d¹⁰ electron configuration and generally only forms bonds involving its s orbital. This means that the highest oxidation state mercury normally attains is II, and for this reason it is generally considered a post-transition metal instead of a transition metal.

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Speculation about higher oxidation states for mercury had been around since the 1970s, and theoretical calculations in the 1990s predicted that it should be stable in the gas phase, with a square-planar geometry consistent with a formal d⁸ configuration. However, experimental proof remained elusive until 2007, when HgF₄ was first prepared using solid neon and argon for matrix isolation at a temperature of 4 K. The compound was detected using infrared spectroscopy. Analysis of density functional theory and coupled cluster calculations showed that the d orbitals are involved in bonding, leading to the suggestion that mercury should be considered a transition metal after all. The predicted bond length in HgF₄ is 188.5 pm, compared with 191.4 pm for HgF₂.

Theoretical studies suggest that mercury is unique among the natural elements of group 12 in forming a tetrafluoride, and attribute this observation to relativistic effects. According to calculations, the tetrafluorides of the "less relativistic" elements cadmium and zinc are unstable and eliminate a fluorine molecule, F₂, to form the metal difluoride. On the other hand, the tetrafluoride of the "more relativistic" synthetic element 112, ununbium, is predicted to be more stable.

References

• Xuefang Wang; Lester Andrews; Sebastian Riedel; and Martin Kaupp (2007). "Mercury Is a Transition Metal: The First Experimental Evidence for HgF₄.". Angew. Chem. Int. Ed. 46 (44): 8371–8375. doi:10.1002/anie.200703710.
• Elusive Hg(IV) species has been synthesized under cryogenic conditions (2007-10-12).
• High Oxidation States: Mercury tetrafluoride synthesized.