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Mercury(IV) fluoride, HgF4, is the first mercury compound to be reported with mercury in the +4
oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...
. Mercury, like the other group 12 elements (
cadmium Cadmium is a chemical element with the Symbol (chemistry), symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12 element, group 12, zinc and mercury (element), mercury. Li ...
and
zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
), has an s2d10
electron configuration In atomic physics and quantum chemistry, the electron configuration is the distribution of electrons of an atom or molecule (or other physical structure) in atomic or molecular orbitals. For example, the electron configuration of the neon ato ...
and generally only forms bonds involving its 6s orbital. This means that the highest oxidation state mercury normally attains is +2, and for this reason it is usually considered a post-transition metal instead of a
transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...
. HgF4 was first reported from experiments in 2007, but its existence remains disputed; experiments conducted in 2008 could not replicate the compound.


History

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 The square planar molecular geometry in chemistry describes the stereochemistry (spatial arrangement of atoms) that is adopted by certain chemical compounds. As the name suggests, molecules of this geometry have their atoms positioned at the corn ...
geometry consistent with a formal d8 configuration. However, experimental proof remained elusive until 2007, when HgF4 was first prepared using solid neon and
argon Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice a ...
for matrix isolation at a temperature of 4  K. The compound was detected using
infrared spectroscopy Infrared spectroscopy (IR spectroscopy or vibrational spectroscopy) is the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection. It is used to study and identify chemical substances or functi ...
. Analysis of density functional theory and
coupled cluster Coupled cluster (CC) is a numerical technique used for describing many-body systems. Its most common use is as one of several post-Hartree–Fock ab initio quantum chemistry methods in the field of computational chemistry, but it is also used in ...
calculations showed that the d orbitals are involved in bonding. This has led to the suggestion that mercury should be considered a transition metal after all (the group 12 metals are sometimes not included as transition metals because they do not have oxidation states beyond +2). 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.


Explanation

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 Relativistic quantum chemistry combines relativistic mechanics with quantum chemistry to calculate elemental properties and structure, especially for the heavier elements of the periodic table. A prominent example is an explanation for the color of ...
. 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.


Synthesis and properties

HgF4 is produced by the reaction of elemental mercury with
fluorine Fluorine is a chemical element with the symbol F and atomic number 9. It is the lightest halogen and exists at standard conditions as a highly toxic, pale yellow diatomic gas. As the most electronegative reactive element, it is extremely reactiv ...
: :Hg + 2 F2 → HgF4 HgF4 is only stable in matrix isolation at ; upon heating, or if the HgF4 molecules touch each other, it decomposes to mercury(II) fluoride and fluorine: :HgF4 → HgF2 + F2 HgF4 is a diamagnetic, square planar molecule. The mercury atom has a formal 6s25d86p6 electron configuration, and as such obeys the octet rule but not the 18-electron rule. HgF4 is isoelectronic with the tetrafluoroaurate anion, , and is valence isoelectronic with the
tetrachloroaurate Chloroauric acid is an inorganic compound with the chemical formula . It forms hydrates . Both the trihydrate and tetrahydrate are known. Both are orange-yellow solids consisting of the planar anion. Often chloroauric acid is handled as a soluti ...
(), tetrabromoaurate (), and tetrachloroplatinate () anions.


References

{{Fluorides Mercury compounds Fluorides Metal halides Hypothetical chemical compounds Substances discovered in the 2000s