HgHg2

HgHg2 is a term used in chemistry to denote two related mercury-containing species whose formulas reflect two mercury atoms together: the neutral diatomic molecule Hg2 and the mercurous ion Hg2^2+. The context determines which form is meant.

The neutral diatomic Hg2 refers to a homonuclear molecule consisting of two mercury atoms bonded together. In the gas phase it can be observed under specialized conditions, and its bond is relatively weak compared with many other diatomic species. The molecule is of interest in spectroscopic studies of mercury bonding, but it is not a major component of common solutions or solids. Its existence highlights mercury’s tendency to form close Hg–Hg interactions under certain energetic conditions.

The mercurous ion, written as Hg2^2+, is a diatomic cation composed of two Hg atoms sharing a bond and carrying an overall +2 charge. In aqueous chemistry, Hg2^2+ is unstable and tends to disproportionate to elemental mercury (Hg^0) and mercuric species (Hg^2+). In practice, mercurous salts form with halides, most notably calomel, Hg2Cl2, where the Hg2^2+ center is paired with chloride ligands. Calomel has historical significance as a medicinal and antiseptic compound and is also encountered in laboratory reagents and certain electrochemical applications.

Safety and environmental concerns are important with any mercury-containing species. Mercury compounds are highly toxic and require careful handling and disposal in accordance with regulatory guidelines.

See also: mercury, mercurous compounds, calomel, diatomic molecules.