Mercurous ion

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.

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.

Hg2+
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.

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