Polycations of mercury

Mercury has the electronic configuration (Xe)4/145d106s2. The first three ionization potentials are 10.43, 18.65 and 34.4 eV, therefore under chemically significant conditions no more than two electrons are removed from the mercury atom. Only one complex of mercury (III), with d9 configuration and a half-life of 5 s at -78 °C, has been synthesized. The synthesis involved electrochemical oxidation of Hg(l,4,8,ll-tetraazacyclotetradecane)(BF4)2 in propiononitrile solution.11 In contrast to most other metals mercury forms polycations, e.g. Hgf+, Hgf+ or Hg3+. 

Table 1 (a) Polycations with Oxidation Numbers Lower Than +1 and (b) Cluster Compounds of Mercury... 

Mercury is the only group 12 member to have a substantial chemistry in the -1-1 oxidation state, in which Hg2 +, the most common of all metal polycations, is formed. This ion was the first one known to exhibit metal-metal bonding. Its chloride salt is the material present in the standard calomel reference electrode. In addition to halides and oxy salts, nitrogen and oxygen-donor ligand complexes of mercury(I) are known, but far fewer than of mercury(II). 

The nitrate, perchlorate, and sulfate salts of mercury(II) are all known. Since mercury(II) is a weak acid, hydrolysis of the ion to form hydroxo-bridged polycations is strongly favored but is inhibited at low pH. In acidic solutions, species such as hydrated mercury(II) and [Hg(N03)]+ are formed. The reaction of mercury(II) with excess nitrate leads to the formation of the complex anion [Hg(N03)4] . This complex is formally eight-coordinate with each nitrate ion coordinating in bidenfate fashion through two O donors see Bidentate Ligand). The average Hg-O bond distance in this complex is 240 pm. 

There are mercury compounds with mercury oxidation numbers lower than +1, e.g. +0.5,16,21 +0.6713,15,27 or +0.35.18,20 Yellow crystals of Hg3(AsF6)2 have been formed by the reaction of metallic mercury with AsFs in liquid S02.13 X-Ray structure determination showed a linear polycation Hg+—Hg—Hg+ with Hg—Hg distances of 255 pm.15 Metallic mercury and SbF5 react in liquid S02 to form Hg3(Sb2Fu)2.15,23 The Hg—Hg distances in the complex Hg3(AlCU)2 are 256 pm 14 the Hg—Cl distances are 251 and 256 pm the Hg—Hg—Hg angle is 174°. Dark red crystals of Hg4(AsF6)2 were obtained in liquid S02. This coordination compound contains centrosymmetric Hg4+ ions, which are connected to chains (see l).21... 

Although no further studies of cationic mercury clusters have been reported, evidence for the formation of cadmium and zinc analogues has been presented. Structural and spectroscopic studies of dehydrated Cd-A (LTA) after reaction with cadmium vapour suggest that various clusters such as Cd ( = 2-4) and Cdj may be present [139-141]. Boddenberg and co-workers [142-144] have examined the reaction of cadmiiun and zinc vapour with H-Y (FAU), and concluded that Cd + or Znf+ polycations are produced. 

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