Mercury polycations
Encyclopedia
Mercury polycations are polyatomic cations that contain only mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...

 atoms. The best known example is the 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 bond 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 formal oxidation 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 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(II) salt, such as
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...

, which causes the Hg(I) salt to completely disproportionate.

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.

Cyclic mercury cations

The triangular cation was confirmed in a reinvestigation of the mineral terlinguaite
Terlinguaite
Terlinguaite is the naturally occurring mineral form of mercury oxychloride with formula Hg2ClO. It is formed by the weathering of other mercury-containing minerals. Discovered in 1900 in the Terlingua District of Brewster County, Texas for which it is named. Its color is yellow, greenish...

 in 1989 and subsequently synthesised in a number of compounds. The bonding has been described in terms of a three-center two-electron bond
Three-center two-electron bond
A three-center two-electron bond is an electron-deficient chemical bond where three atoms share two electrons. The combination of three atomic orbitals form three molecular orbitals: one bonding, one non-bonding, and one anti-bonding. The two electrons go into the bonding orbital, resulting in a...

 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 ), 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
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

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