Mercury(II) hydride (systematically named mercurane(2) and dihydridomercury) is an
inorganic compound with the
chemical formula
In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbol ...
(also written as ). It is both thermodynamically and kinetically unstable at ambient temperature, and as such, little is known about its bulk properties. However, it known as a white, crystalline solid, which is kinetically stable at temperatures below , which was synthesised for the first time in 1951.
Mercury(II) hydride is the second simplest
mercury hydride (after
mercury(I) hydride). Due to its instability, it has no practical industrial uses. However, in
analytical chemistry
Analytical chemistry studies and uses instruments and methods to separate, identify, and quantify matter. In practice, separation, identification or quantification may constitute the entire analysis or be combined with another method. Separati ...
, mercury(II) hydride is fundamental to certain forms of spectrometric techniques used to determine mercury content. In addition, it is investigated for its effect on high sensitivity
isotope-ratio mass spectrometry
Isotope-ratio mass spectrometry (IRMS) is a specialization of mass spectrometry, in which mass spectrometric methods are used to measure the relative abundance of isotopes in a given sample.
This technique has two different applications in the ea ...
methods that involve mercury, such as
MC-ICP-MS, when used to compare thallium to mercury.
Properties
Structure
In solid mercury(II) hydride, the HgH
2 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 A network solid or covalent network solid (also called atomic crystalline solids or giant covalent structures) is a chemical compound (or element) in which the atoms are bonded by covalent bonds in a continuous network extending throughout the ma ...
, solid mercury(II) hydride is a covalently bound
molecular solid
A molecular solid is a solid consisting of discrete molecules. The cohesive forces that bind the molecules together are van der Waals forces, dipole-dipole interactions, quadrupole interactions, π-π interactions, hydrogen bonding, halogen b ...
. This is due to relativistic effects, which also accounts for the relatively low decomposition temperature of -125 °C.
The HgH
2 molecule is linear and symmetric in the form H-Hg-H. The bond length is 1.646543 Å. The antisymmetric stretching frequency, ν
3 of the bond is 1912.8 cm
−1, 57.34473 THz for isotopes
202Hg and
1H.
The energy needed to break the Hg-H bond in HgH
2 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 HgH
2 formation from hydrogen molecules and Hg gas is endothermic at 24.2 kcal/mol.
Biochemistry
Alireza Shayesteh ''et al'' conjectured that bacteria containing the flavoprotein
mercuric reductase, such as ''
Escherichia coli
''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Esc ...
'', can in theory reduce soluble mercury compounds to volatile HgH
2, which should have a transient existence in nature.
Production
Mercury(II) chloride reduction
Mercury(II) hydride may be prepared by the
reduction of
mercury(II) chloride
Mercury(II) chloride (or mercury bichloride, mercury dichloride), historically also known as sulema or corrosive sublimate, is the inorganic chemical compound of mercury and chlorine with the formula HgCl2. It is white crystalline solid and is ...
. In this process, mercury(II) chloride and a hydride salt equivalent react to produce mercury(II) hydride according to the following equations, which depend on the stochiometry of the reaction:
:2 + 2 → +
: + 2 → + 2
Variations of this method exits where mercury(II) chloride is substituted for its heavier halide homologues.
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:
:Hg → Hg
*
:Hg
* + → []
*
:[]
* →
This requires excitation of the mercury atom to the
1P or
3P state, as atomic mercury in its ground-state does not insert into the dihydrogen bond.
Excitation is accomplished by means of an ultraviolet-laser,
or electric discharge.
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:
:2 []
* → 2 HgH +
:2 HgH →
: → 2 Hg +
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 Hg
2H
2).
Reactions
Unpon treatment with a Lewis base, mercury(II) hydride coverts to an adduct. Upon treatment with a standard acid, mercury(II) hydride and its adducts convert either to a mercury salt or a mercuran(2)yl derivative and elemental
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
. Oxidation of mercury(II) hydride gives elemental mercury. Unless cooled below , mercury(II) hydride decomposes to produce elemental mercury and hydrogen:
: → Hg + H
2
History
Mercury(II) hydride was successfully synthesized and identified in 1951 by Egon Wiberg and Walter Henle, by the reaction of mercury(II) iodide and lithium tetrahydroaluminate in a mixture of petroleum ether and tetrahydrofuran. In 1993 Legay-Sommaire announced HgH
2 production in cryogenic argon and krypton matrices with a KrF laser.
In 2004, solid HgH
2 was definitively synthesized and consequentially analysed, by Xuefeng Wang and
Lester Andrews, by direct matrix isolation reaction of excited mercury with molecular hydrogen.
In 2005, gaseous HgH
2 was synthesized by Alireza Shayesteh ''et al'', by the direct gas-phase reaction of excited mercury with molecular hydrogen at standard temperature;
and Xuefeng Wang and Lester Andrews
determined the structure of solid mercury HgH
2, to be a molecular solid.
References
{{Hydrides by group
Mercury(II) compounds
Metal hydrides