Mercury(II) hydride

Mercury(II) hydride (systematically named dihydridomercury) is an inorganic compound with the chemical formula HgH
2. It is a photosensitive, colourless gas, with few other known qualitative descriptors. It has no economical uses, and is produced entirely as an academic curiosity. Mercury(II) hydride is one of the simplest mercury hydrides.

History

It is suspected that in 1951, mercury(II) hydride was synthesised for the first time by Wiberg et al, by the ethereal reaction of mercury(II) iodide and lithium tetrahydroaluminate. In 1993 Legay-Sommaire announced HgH₂ production in cryogenic argon and krypton matrices with a KrF laser.^[1] In 2004, solid HgH₂ was definitively synthesised and consequentially analysed, by Xuefeng Wang and Lester Andrews, by direct matrix isolation reaction of excited mercury with molecular hydrogen.^[2] In 2005, gaseous HgH₂ was synthesised by Alireza Shayesteh et al, by the direct gas-phase reaction of excited mercury with molecular hydrogen at standard temperature;^[3] and Xuefeng Wang and Lester Andrews^[2] determined the structure of solid mercury HgH₂, to be a molecular solid.

Properties

Structure

In solid mercury(II) hydride, the HgH₂ molecules are connected by mercurophilic bonds. Trimers and a lesser proportion of dimers are detected in the vapour. Unlike solid zinc(II), and cadmium(II) hydride, which are network solids, solid mercury(II) hydride is a covalently bound molecular solid. This is due to relativistic effects, which also accounts for the relatively low decomposition temperature of -125°C.^[4]

The HgH₂ molecule is linear and symmetric in the form H-Hg-H. The bond length is 1.646543 Å. The antisymmetric stretching frequency, ν₃ of the bond is 1912.8 cm^-1, 57.34473 THz for isotopes ²⁰²Hg and ¹H.^[4] The energy needed to break the Hg-H bond in HgH₂ is 70 kcal/mol. The second bond in the resulting HgH is much weaker only needing 8.6 kcal/mol to break. Reacting two hydrogen atoms releases 103.3 kcal/mol, and so HgH₂ formation from hydrogen molecules and Hg gas is endothermic at 24.2 kcal/mol.^[4]

Chemical reactions

Mercury(II) hydride undergoes the typical chemical reactions of a molecular metal hydride. Upon treatment with a standard acid, mercury(II) hydride converts to a mercury salt and elemental hydrogen.^{[citation needed]} Oxidation of mercury(II) hydride gives mercury(II) oxide.^{[citation needed]} When heated above −124 °C (−191 °F), it decomposes into elemental mercury and hydrogen:^[2]

HgH
2 → Hg + H₂

Biochemistry

Alireza Shayesteh et al conjectured that bacteria, containing mercuric reductase, such as Escherichia coli, can in theory reduce soluble mercury compounds to volatile HgH₂, which should have a transient existence in nature.

Production

Hydride generation

Mercury(II) hydride can be produced by hydride generation. In this process, mercury(II) and tetrahydroborate or tetrahydroaluminate ions react to produce mercury(II) hydride according to the following equation:

Hg2+
+ 2 MH−
4 → HgH
2 + 2 MH
3 (M = B or Al)

Direct synthesis

Mercury(II) hydride can also be generated by direct synthesis from the elements in the gas phase or in cryogenic inert gas martices:^[4]

Hg → Hg^*

Hg^* + H
2 → [HgH
2]^*

[HgH
2]^* → HgH
2

This requires excitation of the mercury atom to the ¹P or ³P state, as groundstate, atomic mercury does not insert into the dihydrogen bond.^[4] Excitation is accomplished by means of an ultraviolet-laser,^[1] or elecgtric discharge.^[4] The initial yield is high; however, due to the product being in an excited state, a significant amount dissociates rapidly into mercury(I) hydride, then back into the initial reagents:

[HgH
2]^* → HgH + H

HgH → Hg + H

2 H → H
2

This is the preferred method for matrix isolation research. Besides mercury(II) hydride, it also produces other mercury hydrides in lesser quantities, such as the mercury(I) hydrides (HgH and Hg₂H₂).

References