  The appropriateness of including Hg among the transition metals has been
debated for a long time. Although the synthesis of HgF$_{4}$ molecules in gas
phase was reported before, the molecules show strong instabilities and
dissociate. Therefore, the transition metal propensity of Hg remains an open
question. Here, we propose that high pressure provides a controllable method
for preparing unusual oxidation states of matter. Using an advanced structure
search method based on first-principles electronic structure calculations, we
predict that under high pressures, Hg can transfer the electrons in its outmost
$d$ shell to F atoms, thereby acting as a transition metal. Oxidation of Hg to
the +4 state yielded thermodynamically stable molecular crystals consisting of
HgF$_{4}$ planar molecules, a typical geometry for $d^{8}$ metal centers.
