Mercury(I) hydride, a colorless gas, has the formula HgH, containing no Hg-Hg bond. It has not yet been obtained in
bulk; hence its bulk properties are still unknown. Nevertheless, molecular mercury(I) hydrides with the formulae HgH
and Hg2H2 have been separated in solid gas matrices. These molecular hydrides are extremely unstable toward thermal
decomposition. Mercury(I) hydride is the heaviest group 12 monohydride. The composition of mercury(I) hydride is
0.50% hydrogen and 99.50% mercury. In mercury(I) hydride, the formal oxidation states of hydrogen and mercury are
21 and 11, respectively, as the electronegativity of mercury is lower than that of hydrogen. The stability of metal
hydrides with the formula MH (M = Zn-Hg) increases as the atomic number of M increases. The Hg-H bond is very weak
and consequently the compound has only been matrix isolated at temperatures below -267℃ (6K).
