Mercury polycations are polyatomic cations that contain only mercury atoms. The best known example is the Hg
2 ion, found in mercury(I) (mercurous) compounds. The existence of the metal–metal bond in Hg(I) compounds was established using X-ray studies in 1927 and Raman spectroscopy in 1934 making it one of the earliest, if not the first, metal–metal covalent bonds to be characterised.
Other mercury polycations are the linear Hg
3 and Hg
4 ions, and the triangular Hg
3 ion and a number of chain and layer polycations.