Mercury(II) hydride

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Mercury(II) hydride
Names
IUPAC name
Mercury(II) hydride
Other names
Mercurane
Mercuric hydride
Identifiers
Jmol-3D images Image
Properties
HgH
2
Molar mass 202.61 g mol−1
Related compounds
Related compounds
Zinc hydride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Mercury(II) hydride (systematically named mercurane(2) and dihydridomercury) is an inorganic compound with the chemical formula HgH
2
(also written as [HgH
2
]
). Bulk mercury(II) hydride is a white solid, which is stable up to −125 °C (−193 °F). The vapour is photosensitive and colourless, with few other known qualitative descriptors. It has no economic uses, and is produced entirely as an academic curiosity.

History[edit]

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.

Chemical properties[edit]

Acidity[edit]

The two-coordinate hydridomercury group (-HgH) in hydridomercury complexes such as mercury(II) hydride can accept an electron-pair donating ligand into the molecule by adduction:[2]

[HgH
2
]
+ L → [HgH
2
L]

Because of this acceptance of the electron-pair donating ligand (L), mercury(II) hydride has Lewis-acidic character.

Structure[edit]

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[edit]

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[edit]

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[edit]

Hydride generation[edit]

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[edit]

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[edit]

  1. ^ a b Legay-Sommaire, N.; F. Legay (1993). "Photochemistry in Hg doped matrices. Infrared spectra of mercury hydrides: HgH2, HgD2, HHgD, HgD". Chemical Physics Letters 207 (2-3): 123–128. doi:10.1016/0009-2614(93)87001-j. ISSN 0009-2614. 
  2. ^ a b c d Wang, Xuefeng; Andrews, Lester (2005). "Mercury dihydride forms a covalent molecular solid". Physical Chemistry Chemical Physics 7 (5): 750. doi:10.1039/b412373e. ISSN 1463-9076. 
  3. ^ Shayesteh, Alireza; Yu, Shanshan; Bernath, Peter F. (2005). "Infrared Emission Spectra and Equilibrium Structures of Gaseous HgH2and HgD2". The Journal of Physical Chemistry A 109 (45): 10280–10286. doi:10.1021/jp0540205. ISSN 1089-5639. 
  4. ^ a b c d e f Shayesteh, Alireza; Shanshan Yu; Peter F. Bernath (2005). "Gaseous HgH2, CdH2, and ZnH2". Chemistry - A European Journal 11 (16): 4709–4712. doi:10.1002/chem.200500332. ISSN 0947-6539.