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.
The best known polycation of mercury is Hg2+
2, in which mercury has a formal oxidation state of +1. The Hg2+
2 ion was perhaps the first metal-metal bonded species confirmed. The presence of the Hg2+
2 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 Hg2+
2 units in the solid state first determined in 1926 using X-Ray diffraction. The presence of the metal-metal bond in solution was confirmed using Raman spectroscopy in 1934.
2 is stable in aqueous solution, where it is in equilibrium with Hg2+
and elemental Hg, with Hg2+
present at around 0.6%. This equilibrium is readily shifted by the addition of an anion which forms an insoluble Hg(II) salt, such as S2−
, which causes the Hg(I) salt to completely disproportionate, or by the addition of an anion which forms an insoluble Hg(I) salt, such as Cl−
, which causes the elemental mercury and Hg2+ to completely recombine into the mercury(I) salt.
Linear trimercury and tetramercury cations
Compounds containing the linear Hg2+
3 (mercury(2⁄3)) and Hg2+
4 (mercury(1⁄2)) cations have been synthesised. These ions are only known in the solid state in compounds such as Hg
2 and Hg
2. The Hg–Hg bond length is 255 pm in Hg2+
3, and 255–262 pm in Hg2+
4. The bonding involves 2-centre-2-electron bonds formed by 6s orbitals.
Cyclic mercury cations
The triangular Hg4+
3 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 (in D3h symmetry) a bonding "a1" orbital.
Chain and layer polycations
The golden yellow compound Hg
6), named "alchemists' gold" by its discoverers, contains perpendicular chains of Hg atoms.
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