Mercury polycations are polyatomiccations that contain only mercuryatoms. The best known example is the ion, found in mercury compounds. The existence of the metal–metal bond in Hg 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 and ions, and the triangular ion and a number of chain and layer polycations.
Mercury(I)
The best known polycation of mercury is, in which mercury has a formaloxidation state of +1. The ion was perhaps the first metal-metal bonded species confirmed. The presence of the ion in solution was shown by Ogg in 1898. In 1900, Baker showed the presence of HgCl dimers in the vapour phase. The presence of units in the solid state was first determined in 1926 using X-ray diffraction. The presence of the metal-metal bond in solution was confirmed using Raman spectroscopy in 1934. is stable in aqueous solution, where it is in equilibrium with and elemental Hg, with present at around 0.6%. This equilibrium is readily shifted by the addition of an anion which forms an insoluble Hg salt, such as sulfide|, which causes the Hg salt to completely disproportionate, or by the addition of an anion which forms an insoluble Hg salt, such as chloride|, which causes the elemental mercury and Hg2+ to completely recombine into the mercury salt. Minerals that are known that contain the cation include eglestonite.
Linear trimercury and tetramercury cations
Compounds containing the linear ) and ) cations have been synthesised. These ions are only known in the solid state in compounds such as and. The Hg–Hg bond length is 255 pm in, and 255–262 pm in. The bonding involves 2-centre-2-electron bonds formed by 6s orbitals.
The triangular cation was confirmed in a reinvestigation of the mineral terlinguaite in 1989 and subsequently synthesised in a number of compounds. The bonding has been described in terms of a three-center two-electron bond where overlap of the 6s orbitals on the mercury atoms gives a bonding "a1" orbital.
Chain and layer polycations
The golden yellow compound ), named "alchemists' gold" by its discoverers, contains perpendicular chains of Hg atoms. The "metallic" compounds and contain hexagonal layers of mercury atoms separated by layers of anions. They are both superconductors below 7 K.
