Mercury, covalent bonding

The biochemical basis for the toxicity of mercury and mercury compounds results from its ability to form covalent bonds readily with sulfur. Prior to reaction with sulfur, however, the mercury must be metabolized to the divalent cation. When the sulfur is in the form of a sulfhydryl (— SH) group, divalent mercury replaces the hydrogen atom to form mercaptides, X—Hg— SR and Hg(SR)2, where X is an electronegative radical and R is protein (36). Sulfhydryl compounds are called mercaptans because of their ability to capture mercury. Even in low concentrations divalent mercury is capable of inactivating sulfhydryl enzymes and thus causes interference with cellular metaboHsm and function (31—34). Mercury also combines with other ligands of physiological importance such as phosphoryl, carboxyl, amide, and amine groups. It is unclear whether these latter interactions contribute to its toxicity (31,36). 

Mercury has a characteristic ability to form not only conventional ammine and amine complexes but also, by the displacement of hydrogen, direct covalent bonds to nitrogen, e.g. ... 

Hg22+ is a polyatomic cation there is a covalent bond between the mercury atoms. 

In compounds, mercury has the oxidation number +1 or +2. Its compounds with oxidation number +1 are unusual in that the mercury(I) cation is the covalently bonded diatomic ion (Hg—Hg)2+, written Hg22+. 

Like mercury, tin is a metal that has a tendency to form covalent bonds with organic groups. The compounds to be discussed here are tributyl derivatives of tetravalent tin. The general formula for them is... 

Opinions differ on the nature of the metal-adsorbed anion bond for specific adsorption. In all probability, a covalent bond similar to that formed in salts of the given ion with the cation of the electrode metal is not formed. The behaviour of sulphide ions on an ideal polarized mercury electrode provides evidence for this conclusion. Sulphide ions are adsorbed far more strongly than halide ions. The electrocapillary quantities (interfacial tension, differential capacity) change discontinuously at the potential at which HgS is formed. Thus, the bond of specifically adsorbed sulphide to mercury is different in nature from that in the HgS salt. Some authors have suggested that specific adsorption is a result of partial charge transfer between the adsorbed ions and the electrode. 

Ans. The actual formula implies that the two mercury atoms are covalently bonded together. 

Formula HgCb MW 271.50 covalent bonding, shghtly ionized in water Synonyms mercuric chloride mercury bichloride corrosive subhmate mercury perchloride... 

Formula (CH3)2Hg MW 230.67 covalent bonding of methyl radicals to mercury atom linear shape Synonyms dimethylmercury methylmercury... 

The second group consists of compounds in which covalent bonding is important. These compounds tend to melt with Ihe formation of discrete molecules although autoionizalion may occur. For example, the mercury(H) halides ionize as follows ... 

The solvent influences on the complex formation and stability have been reviewed by Golub et al.194 Several monomeric complexes of Hg(SCN)2 with N, O, P, As and S donor ligands are known with terminal Hg—SCN bonds.224-2 Some thiocyanate-bridged dimeric complexes of mercury(II) are also known.225,226 Recently the isolation of a mercury(II) thiocyanate complex with hexamethylenetetramine with exclusively N-bonded SCN groups has been published (Figure 9).233 The compounds (CH2)6N4-Hg(SCN)2 and (CH2)6N4-2Hg(SCN)2 exhibit covalently bonded Hg—s.394,595... 

The biochemical basis for the toxicity of mercury and mercury compounds resulls from its ability to form covalent bonds with sulfur. Even In low coiiccninilinns divalent mercury is capable of inaelivaiing enzymes containing suirhydrvl I —Nil) groups, causing iiileil crcncc with cellular metabolism and function. 

Literally hundreds of complex equilibria like this can be combined to model what happens to metals in aqueous systems. Numerous speciation models exist for this application that include all of the necessary equilibrium constants. Several of these models include surface complexation reactions that take place at the particle-water interface. Unlike the partitioning of hydrophobic organic contaminants into organic carbon, metals actually form ionic and covalent bonds with surface ligands such as sulfhydryl groups on metal sulfides and oxide groups on the hydrous oxides of manganese and iron. Metals also can be biotransformed to more toxic species (e.g., conversion of elemental mercury to methyl-mercury by anaerobic bacteria), less toxic species (oxidation of tributyl tin to elemental tin), or temporarily immobilized (e.g., via microbial reduction of sulfate to sulfide, which then precipitates as an insoluble metal sulfide mineral). 

Following the preparation of 4.79, a number of other cyclic mercury crown compounds have been synthesised, which do exhibit halide complexation behaviour. Compound 4.80, forms a 1 1 polymer with bromide in the solid state in which the Hr anions perch above the Hg3 plane. The Hg—Br distances of 3.07-3.39A are considerably longer than normal Hg—Br covalent bonds (about 2.54A).61 The compound also binds SCN- with similarly long bonds as shown in Figure 4.34.60 The analogous chloride complex has a 3 2 stoichiometry suggesting a triple-decker sandwich of type [4.80 Cl 4.80 Cl 4.80]2. ... 

A new, more accurate electron diffraction study of gaseous mercuric chloride has been reported.159 The interatomic distances (Hg—Cl = 2.25 A, Cl—Cl = 4.48 A) are shorter than previously reported values by 0.02 to 0.09 A. A complete normal-co-ordinate analysis of bis(methylthio)mercury has also been reported.160 The Raman spectra of gaseous mercuric chloride, bromide, and iodide have been reported.161 The bond polarizability derivatives calculated from the data increase in the order Cl < Br < I, suggesting an increased degree of covalence in the mercury-halogen bond with increasing size of the halogen atom. 

Nitric Oxide (-NO). Reduction of the oxides of nitrogen (-NO, -N02, and N20) usually involves the addition of hydrogen atoms that are electrogenerated. Figure 11.6 illustrates the chronopotentiometric reduction of -NO in aqueous media at pH 7.0 and pH 5.0.9 The use of a mercury electrode inhibits the reduction of H30+ to H2 (is0, —2.2 V vs. SCE at pH 5), but allows formation of H when it couples with a substrate via covalent-bond formation ... 

For simplicity, mercuric acetate is shown with covalent bonds. Mercury has a filled 5d subshell. Two of these 5d unshared electrons are shown. 

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