Mercury tetrafluoride

Mercury(IV) fluoride

Identifiers
Properties
Chemical formula	HgF4
Molar mass	276.58 g/mol
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Y  (what is YN ?)
Infobox references

Mercury(IV) fluoride, HgF₄, is the first mercury compound to be reported in 2007 with mercury in the oxidation state IV. Mercury, like the other group 12 elements (cadmium and zinc), has an s²d¹⁰electron configuration and generally only forms bonds involving its 6s orbital. This means that the highest oxidation state mercury normally attains is II, and for this reason it is usually considered a post-transition metal instead of a transition metal. The existence of the compound is disputed; experiments conducted in 2008 could not replicate the existence of HgF₄.

Speculation about higher oxidation states for mercury had existed 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. However, that conclusion has been challenged by W. B. Jensen with the argument that HgF₄ only exists under highly atypical non-equilibrium conditions and should best be considered as an exception.

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, copernicium, is predicted to be more stable. However, more recent theoretical studies cast doubt on the possible existence of mercury(IV) and even copernicium(IV) fluoride.