Mercury polycations are polyatomic

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

s that contain only mercury 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 Raman spectroscopy () (named after physicist C. V. Raman) is a Spectroscopy, spectroscopic technique typically used to determine vibrational modes of molecules, although rotational and other low-frequency modes of systems may also be observed. Ra ...

in 1934 making it one of the earliest, if not the first, metal–metal

covalent bond A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atom ...

s 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 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%. Anions of insoluble salts readily shift the equilibrium: , which forms an insoluble Hg(II) salt, induces complete disproportionation, whereas , which forms an insoluble Hg(I) salt, induces the reverse. Most salts with

main group element In chemistry and atomic physics, the main group is the group (periodic table), group of chemical element, elements (sometimes called the representative elements) whose lightest members are represented by helium, lithium, beryllium, boron, carbon ...

s tend to contain only Hg(II) and metallic mercury, because the presence of strong Lewis bases destabilizes the intermetallic bond. In appropriate solvents, however, Hg(I) salts with derivatives of

amide In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a chemical compound, compound with the general formula , where R, R', and R″ represent any group, typically organyl functional group, groups or hydrogen at ...

pyridine Pyridine is a basic (chemistry), basic heterocyclic compound, heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom . It is a highly flammable, weak ...

s, phosphorus trifluoride,

tin(II) Tin is a chemical element; it has symbol Sn () and atomic number 50. A silvery-colored metal, tin is soft enough to be cut with little force, and a bar of tin can be bent by hand with little effort. When bent, a bar of tin makes a sound, the ...

, and certain other main group elements are all known. Minerals that are known that contain the cation include eglestonite.

Linear trimercury and tetramercury cations

Compounds containing the linear (in which mercury has the formal oxidation state ) and (in which mercury has the formal oxidation state ) 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 is the naturally occurring mineral with formula . It is formed by the weathering of other mercury (element), mercury-containing minerals. It was discovered in 1900 in the Terlingua, Texas, Terlingua District of Brewster County, Texa ...

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 D_3h 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.

References

{{Mercury compounds Mercury compounds Cations