Mercury polycations

Mercury polycations are polyatomic cations that contain only mercury atoms. The best known example is the Hg2+
2 ion, found in mercury(I) (mercurous) compounds. The existence of the metal–metal bond in Hg(I) compounds was established using X-ray studies in 1927[1] and Raman spectroscopy in 1934[2] making it one of the earliest, if not the first, metal–metal covalent bonds to be characterised.

Other mercury polycations are the linear Hg2+
3 and Hg2+
4 ions,[2] and the triangular Hg4+
3 ion [3] and a number of chain[4] and layer polycations.[5]

Mercury(I)

The best known polycation of mercury is Hg2+
2, in which mercury has a formal oxidation state of +1. The Hg2+
2 ion was perhaps the first metal-metal bonded species confirmed. The presence of the Hg2+
2 ion in solution was shown by Ogg in 1898.[6] In 1900, Baker showed the presence of HgCl dimers in the vapour phase.[7] The presence of Hg2+
2 units in the solid state was first determined in 1926 using X-ray diffraction.[1] The presence of the metal-metal bond in solution was confirmed using Raman spectroscopy in 1934.[2]

Hg2+
2 is stable in aqueous solution, where it is in equilibrium with Hg2+
 and elemental Hg, with Hg2+
 present at around 0.6%.[2] This equilibrium is readily shifted by the addition of an anion which forms an insoluble Hg(II) salt, such as S2−
, which causes the Hg(I) salt to completely disproportionate, or by the addition of an anion which forms an insoluble Hg(I) salt, such as Cl−
, which causes the elemental mercury and Hg2+ to completely recombine into the mercury(I) salt.[2]

Minerals that are known that contain the Hg2+
2 cation include eglestonite.[8]

Linear trimercury and tetramercury cations

Compounds containing the linear Hg2+
3 (mercury(​2⁄3)) and Hg2+
4 (mercury(​1⁄2)) cations have been synthesised. These ions are only known in the solid state in compounds such as Hg
3(AlCl
4)
2 and Hg
4(AsF
6)
2.[2] The Hg–Hg bond length is 255 pm in Hg2+
3, and 255–262 pm in Hg2+
4.[2] The bonding involves 2-centre-2-electron bonds formed by 6s orbitals.[2]

Cyclic mercury cations

The triangular Hg4+
3 cation was confirmed in a reinvestigation of the mineral terlinguaite in 1989[3] and subsequently synthesised in a number of compounds.[9] The bonding has been described in terms of a three-center two-electron bond where overlap of the 6s orbitals on the mercury atoms gives (in D3h symmetry) a bonding "a1" orbital.[10]

Chain and layer polycations

The golden yellow compound Hg
2.86(AsF
6), named "alchemists' gold" by its discoverers,[4] contains perpendicular chains of Hg atoms.

The "metallic" compounds Hg
3NbF
6 and Hg
3TaF
6 contain hexagonal layers of mercury atoms separated by layers of MF−
6 anions.[5] They are both superconductors below 7 K.[11]

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References
  1. Wells, A. F. (1962). Structural Inorganic Chemistry (3rd ed.). Oxford Science Publications.
  2. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  3. Terlinguait, Hg4O2Cl2 - ein Mineral mit ungewöhnlichen Hg3-Baueinheiten, K. Brodersen,G. Göbel, G. Liehr, Zeitschrift für anorganische und allgemeine Chemie, 575, 1, 1989, 145 - 153, doi:10.1002/zaac.19895750118
  4. Alchemists' Gold, Hg2.86 AsF6; An X-Ray Crystallographic Study of A Novel Disordered Mercury Compound Containing Metallically Bonded Infinite Cations, I. David Brown, Brent D. Cutforth, Colin G. Davies, Ronald J. Gillespie, Peter R. Ireland, and John E. Vekris, Can. J. Chem. 52(5): 791-793 (1974),doi:10.1139/CJC-52-5-791
  5. Brown, I. D.; Gillespie, R. J.; Morgan, K. R.; Tun, Z.; Ummat, P. K. (1984). "Preparation and crystal structure of mercury hexafluoroniobate (Hg
    3    NbF
    6    ) and mercury hexafluorotantalate (Hg
    3    TaF
    6    ): mercury layer compounds". Inorganic Chemistry. 23 (26): 4506–4508. doi:10.1021/ic00194a020.
  6. A. Ogg; Zeitschrift Physische Chemie 27, 285 (1898)
  7. Vapour density of dried mercurous chloride, H. Brereton Baker M.A., J. Chem. Soc., Trans., 1900, 77, 646, doi:10.1039/CT9007700646
  8. Eglestonite, [Hg2]3Cl3O2H: Confirmation of the chemical formula by neutron powder diffraction, Mereiter K., Zemann J., Hewatt A.W. American Mineralogist, 77, (1992), 839-842
  9. [Hg3]4+ Cation in Inorganic Crystal Structures, S. V. Borisov, S. A. Magarill and N. V. Pervukhina, Journal of Structural Chemistry, 44, 3, 2003, 441-447, doi:10.1023/B:JORY.0000009672.71752.68
  10. Synthesis and crystal structure of the subvalent mercury cluster [triangulo-Hg3(-dmpm)4][O3SCF3]4 (dmpm = Me2PCH2PMe2), Anna Mühlecker-Knoepfler, Ernst Ellmerer-Müller, Robert Konrat, Karl-Hans Ongania, Klaus Wurst and Paul Peringer, J. Chem. Soc., Dalton Trans., 1997, 1607 - 1610, doi:10.1039/a700483d
  11. Superconductivity of Hg3NbF6 and Hg3TaF6,(1983),W. R. Datars, K. R. Morgan and R. J. Gillespie, Phys. Rev. B 28, 5049 - 5052, doi:10.1103/PhysRevB.28.5049
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