The prediction of compounds whose elements are found in high oxidation states beyond the naturally occurring forms is a very exciting topic in theoretical condensed matter. Group IIB elements, including Zn, Cd, and Hg, are usually defined as post-transition metals because they are commonly oxidized only to the +2 state. However, it is possible that Hg could be stable in higher oxidation states owing to the large relativistic effects that perturb the position of the 5d energies.
Metastable, gas-phase planar molecules containing Hg-IV have been predicted by quantum chemistry calculations. The previous studies predicted that HgF4 molecules could resist dissociation to HgF2 and F2 molecules.[1-3] HgF4 molecules has also been detected using matrix-isolation infrared (IR) spectroscopy. Regarding Hg-III, it has been reported experimentally in a short-lived compound. A Hg-III fluoride in molecular form has been recently studied by quantum chemistry calculations. All these results clearly reveal the propensity of Hg to act as a transition metal while reacting with strong oxidizing agents, such as fluorine. However, Hg in a thermodynamically stable high-oxidation state, such as in HgF4, has not been achieved.
Recently, a group of CSRC researchers has found that high-pressure techniques are a viable method to stabilize both Hg-IV and Hg-III oxidation states. Using an advanced structure search algorithm based on Particle Swarm Optimization algorithm  and first principles electronic structure calculations, they found that under high pressure Hg in Hg-F compounds transfers charge from their d-orbitals to the F, thus behaving as a transition metal. Oxidizing Hg to +4 and +3 yielded the thermodynamically stable compounds HgF4 and HgF3. The former consists of HgF4 planar molecules, a typical geometry for d8 metal centers. HgF3 is metallic and ferromagnetic owing to the d9 configuration of Hg, with a large gap between its partially occupied and unoccupied bands under high pressure, which suggests properties analogous to a transparent conductor.
Fig. 1. Real-space configuration of the crystalline structures: For HgF4, a) side view and b) top views of the thermodynamically unstable Pca21 low-pressure structure and (c) and (d) are the same views for the stable high-pressure structure I4m. For HgF3: e) is the low-pressure C2m and f) is Fm-3m.
Fig.2. (a) Plot of the ELF of the HgF3 structure. ELF ranges from 0.15 (blue) to 0.85 (red); (b) Plot of the band structure of HgF3 at 200 GPa, separated for the majority (spin up) and minority (spin down) electrons; (c) Projected DOS of HgF3 at 200 GPa, where sign indicate majority-minority electrons. The black and cyan lines and the magenta line with shaded area show the PDOS of Hg 5d, F1 2p and F2 2p states. respectively.
For more information, please see the paper: “Mercury under Pressure acts as a Transition Metal: Calculated from First Principles”, Angew. Chem. Int. Ed. 54, 9280 (2015), (Pub. 3 Jul. 2015). DOI: 10.1002/anie.201505500.
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