Mercury polycations are

polyatomic A polyatomic ion, also known as a molecular ion, is a covalent bonded set of two or more atoms, or of a complex (chemistry), metal complex, that can be considered to behave as a single unit and that has a net electrical charge, charge that is no ...

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 conven ...

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 Indian physicist C. V. Raman) is a spectroscopic technique typically used to determine vibrational modes of molecules, although rotational and other low-frequency modes of systems may also be observed. Raman ...

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 ato ...

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 of elements (sometimes called the representative elements) whose lightest members are represented by helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, and fluorine as arran ...

s tend to contain only Hg(II) and metallic mercury, because the presence of strong

Lewis base A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...

s 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 compound with the general formula , where R, R', and R″ represent organic groups or hydrogen atoms. The amide group is called a peptide bond when it i ...

pyridine Pyridine is a basic 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, weakly alkaline, water-miscible liquid w ...

phosphorus trifluoride Phosphorus trifluoride (formula P F3), is a colorless and odorless gas. It is highly toxic and reacts slowly with water. Its main use is as a ligand in metal complexes. As a ligand, it parallels carbon monoxide in metal carbonyls, and indeed ...

tin(II) Tin is a chemical element with the symbol Sn (from la, stannum) and atomic number 50. Tin is a silvery-coloured 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, t ...

, 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 (mercury()) and (mercury()) 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 Hg2 Cl O. It is formed by the weathering of other mercury-containing minerals. It was discovered in 1900 in the Terlingua District of Brewster County Brewster County is a county l ...

in 1989 and subsequently synthesised in a number of compounds. The bonding has been described in terms of a

three-center two-electron bond A three-center two-electron (3c–2e) 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''-b ...

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