Mercury divalent compounds

Mercury and its inorganic divalent compounds criteria document for an OEL... 

Yang and Zhu [107] have studied, applying several electrochemical methods and mercury electrodes, electrochemical behavior of pharmaceutically important dipeptide captopril. In acidic solution, one-electron transfer led to the formation of a univalent mercury-sulfur compound, which was strongly adsorbed at the electrode surface and gradually transformed into the divalent mercury-sulfur compound. 

The apparent anomaly between mercury and the lighter elements of transition group 2. in that mercury regularly forms both univalent and divalent compounds, while zinc and cadmium do so very rarely, is partly under mm id from the observation that mercury III salts ionize even in the gaseous late to Hg.. rather than Hg Evidence for this double ion is provided by its Hainan spectral line, by the lineal CI-Hg-Hg-CI units in crystals or mercury It chloride, and by the cml of incrciirytll nitrate concentration cells The anomaly is fuitlicr removed by the obsetv.ttioii that cadmium also forms a (much less stable) diatomic ton Cdj T eg., ill Cd.-lAICL) . 

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). 

Toxicity. The toxic el frets of mercury and mercury compounds are well known, and several detailed discussions on mercury toxicity are available. Toxicity lo the central nervous system is more prominent aftci exposure lo mercury vapor than lo divalent mercury. 

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. 

In the gas phase, divalent mercury has been shown to be linear and therefore to be sp hybridized. However, in solution the X—R—X, R—Hg—X, or R—Hg—R bond angle in divalent mercury compounds varies from 130 to 180°. The variation in geometry is not yet entirely understood, so we shall follow Jensen s example and assume that, even in solution, divalent mercury is sp hybridized and that if a divalent mercury compound donates one empty orbital to coordinate with a Lewis base it rehybridizes to sp2 (F. R. Jensen and B. Rickborn, Electrophilic Substitution of Organomercurials, pp. 35, 36). 

Photochemical behavior of monosilanes has been investigated by mercury-sensitized photolysis (15-19), flash photolysis (20, 21), vacuum ultraviolet photolysis (22-27), and matrix photolysis (28-30). The first examples of the photolysis of permethylated polysilanes were published in 1970 (14). All of the cyclic and acyclic permethylpolysilanes with the exception of hexamethyldisilane readily undergo photolysis on irradiation with ultraviolet light to give shorter chain compounds with the concurrent generation of the divalent silicon intermediate, dimethylsilylene (8). 

A summary of the thermodynamic properties of elemental mercury and its compounds has appeared. Although Hg is considered a class b or soft metal ion see Class A Class B Behavior and Hard Soft Acids and Bases), its coordination behavior suggests that it functions as a harder acid than the divalent ion. Advanced texts have discussed the chemistry of the element and its compounds. ... 

A high affinity of polyvalent metal phosphates for divalent cations is not an unusual fact [1]. There are dozens of known exchangers with much higher capacities than that found for the 1. Nevertheless, this layered compound is an extremely interesting ion-exchanger. He shows a uniquely high affinity for the mercury(II) ion, while all other known titanium phosphate or titanium arsenate sorbents do not exhibit any preference for the Hg cation. 

Mercury is a metal element that occurs naturally in the environment. Metallic or elemental mercury (Hg°) is the main form of mercury released into the air by natural processes. Mercury bound to other chemicals may have valence states of either +1 (Hg+1) or +2 (Hg+2). Mercury with a valence state of+1 is referred to as mercurous mercury, and mercury with a valence state of +2 is referred to as mercuric mercury. Many inorganic and organic compounds of mercury can be formed from the mercuric (divalent) cation (Hg+2). For information on the physical and chemical properties of mercury, refer to Chapter 3. 

Once absorbed, metallic and inorganic mercury enter an oxidation-reduction cycle. Metallic mercury is oxidized to the divalent inorganic cation in the red blood cells and lungs of humans and animals. Evidence from animal studies suggests that the liver is an additional site of oxidation. Absorbed divalent cation from exposure to mercuric compounds can, in turn, be reduced to the metallic or monovalent form and released as exhaled metallic mercury vapor. In the presence of protein sulfhydryl groups, mercurous mercury (Hg+) disproportionates to one divalent cation (Hg+2) and one molecule at the zero oxidation state (Hg°). The conversion of methylmercury or phenylmercury into divalent inorganic mercury can probably occur soon after absorption, also feeding into the oxidation-reduction pathway. 

The divalent cation exists in both a nondiffusible form (tissues) and a diffusible form (blood) (Halbach and Clarkson 1978 Magos 1967) (see Section 2.3.2). The mechanism for the distribution of mercury and its compounds probably depends on the extent of uptake of the diffusible forms into different tissues or on the mercury-binding to protein-binding sites (sulfhydryl groups) in red cells and plasma proteins (Clarkson 1972b). 

Mechanisms for the toxic effects of inorganic and organic mercury are believed to be similar. It has been suggested that the relative toxicities of the different forms of mercury (e g., metallic, monovalent, and divalent cations and methyl- and phenylmercury compounds) are related, in part, to its differential accumulation in sensitive tissues. This theory is supported by the observation that mercury rapidly accumulates in the kidneys and specific areas of the central nervous system (Rothstein and Hayes 1960 Somjenetal. 1973). 

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