Mercury(IV) fluoride

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Square planar structure of HgF₄.

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.

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 complex. On the other hand, the tetrafluoride of the "more relativistic" synthetic element 112, ununbium, is predicted to be more stable.