Although the Group 12 elements are generally considered to be post-transition metals with filled d shells, quantum chemical calculations predict a stable HgF₄ square-planar d⁸ complex due to relativistic stabilization of Hgᴵⱽ relative to Hgᴵᴵ. In its tetravalent oxidation state, mercury is a true transition metal that utilizes 5d orbitals fully in bonding. The first experimental evidence is reported for HgᴵⱽF₄ in solid neon by IR spectroscopy, after reaction of mercury with excess fluorine under mercury arc irradiation and subsequent annealing. Assignment of the new bands to HgF₄ is supported by coupled-cluster and density functional calculations of structures and vibrational frequencies.