Mercury polycations
Mercury polycations are polyatomic cations that contain only mercury atoms. The best known example is the Template:Chem/link 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[2]Script error: No such module "Unsubst". and Raman spectroscopy in 1934[3] making it one of the earliest, if not the first, metal–metal covalent bonds to be characterised.
Other mercury polycations are the linear Template:Chem/link and Template:Chem/link ions,[3] and the triangular Template:Chem/link ion [4] and a number of chain[5] and layer polycations.[6]
Mercury(I)
The best known polycation of mercury is Template:Chem/link, in which mercury has a formal oxidation state of +1. The Template:Chem/link ion was perhaps the first metal-metal bonded species confirmed. The presence of the Template:Chem/link ion in solution was shown by Ogg in 1898.[7] In 1900, Baker showed the presence of HgCl dimers in the vapour phase.[8] The presence of Template:Chem/link units in the solid state was first determined in 1926 using X-ray diffraction.[2] The presence of the metal-metal bond in solution was confirmed using Raman spectroscopy in 1934.[3]
Template:Chem/link is stable in aqueous solution, where it is in equilibrium with Template:Chem/link and elemental Hg, with Template:Chem/link present at around 0.6%. Anions of insoluble salts readily shift the equilibrium: [[sulfide|Template:Chem/link]], which forms an insoluble Hg(II) salt, induces complete disproportionation, whereas [[chloride|Template:Chem/link]], which forms an insoluble Hg(I) salt, induces the reverse.[3] Most salts with main group elements tend to contain only Hg(II) and metallic mercury, because the presence of strong Lewis bases destabilizes the intermetallic bond. In appropriate solvents, however, Hg(I) salts with derivatives of amides, pyridines, phosphorus trifluoride, tin(II), and certain other main group elements are all known.[9]
Minerals that are known that contain the Template:Chem/link cation include eglestonite.[10]
Linear trimercury and tetramercury cations
Compounds containing the linear Template:Chem/link (in which mercury has the formal oxidation state <templatestyles src="Fraction/styles.css" />2⁄3) and Template:Chem/link (in which mercury has the formal oxidation state <templatestyles src="Fraction/styles.css" />1⁄2) cations have been synthesised. These ions are only known in the solid state in compounds such as Template:Chem/link and Template:Chem/link. The Hg–Hg bond length is 255 pm in Template:Chem/link, and 255–262 pm in Template:Chem/link. The bonding involves 2-centre-2-electron bonds formed by 6s orbitals.[3]
Cyclic mercury cations
The triangular Template:Chem/link cation was confirmed in a reinvestigation of the mineral terlinguaite in 1989[4] and subsequently synthesised in a number of compounds.[11] 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.[12]
Chain and layer polycations
The golden yellow compound Template:Chem/link), named "alchemists' gold" by its discoverers,[5] contains perpendicular chains of Hg atoms.
The "metallic" compounds Template:Chem/link and Template:Chem/link contain hexagonal layers of mercury atoms separated by layers of Template:Chem/link anions.[6] They are both superconductors below 7 K.[13]
References
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- ↑ A. Ogg; Zeitschrift Physische Chemie 27, 285 (1898)
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- ↑ 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
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