Mercury polycations are polyatomic cations that contain only mercury atoms. The best known example is the Hg2+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, covalent metal-metal bonds to be characterised. Other mercury polycations are the linear Hg2+3 and Hg2+4 ions, and the triangular Hg4+3 ion and a number of chain and layer polycations.