Mercury Lewis base

Many years ago, geochemists recognized that whereas some metallic elements are found as sulfides in the Earth s crust, others are usually encountered as oxides, chlorides, or carbonates. Copper, lead, and mercury are most often found as sulfide ores Na and K are found as their chloride salts Mg and Ca exist as carbonates and Al, Ti, and Fe are all found as oxides. Today chemists understand the causes of this differentiation among metal compounds. The underlying principle is how tightly an atom binds its valence electrons. The strength with which an atom holds its valence electrons also determines the ability of that atom to act as a Lewis base, so we can use the Lewis acid-base model to describe many affinities that exist among elements. This notion not only explains the natural distribution of minerals, but also can be used to predict patterns of chemical reactivity. 

Metals that are soft Lewis acids, for example cadmium, mercury, and lead, are extremely hazardous to living organisms. Tin, in contrast, is not. One reason is that tin oxide is highly insoluble, so tin seldom is found at measurable levels in aqueous solution. Perhaps more important, the toxic metals generally act by binding to sulfur in essential enz Tnes. Tin is a harder Lewis acid than the other heavy metals, so it has a lower affinity for sulfur, a relatively soft Lewis base. 

The reaction between dimercury(I) salts and molecules with an electron-pair-donating atom normally destroys the metal-metal bond of the dimercury(I) ion Hg+—Hg+ by disproportionation, forming metallic mercury and a mercury(II) compound, but the use of nonpolar solvents, weak Lewis bases and dialytic crystallization methods has contributed to the successful preparation of single crystals of several dimercury(I) coordination compounds in the past 25 years.9,30,31 The myth that the dimercury(I) species Hg + forms few coordination compounds has been exploded. 

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). 

Mercury(II), Hg , is a soft Lewis acid, and so is found in nature only combined with soft Lewis bases, the most common of which is S ". Sulfide can readily and permanently abstract Hg from its complexes with harder bases in ore-forming geological reaction mixtures. Zinc(II), which exhibits borderline behaviour, is harder and forms... 

Mercury(II) is a very soft Lewis acid, which forms stable complexes preferentially with soft Lewis bases such as sulfur ligands. You should remember here that the major natural form of mercury is sulfides. Increasing the pH of the aqueous solution due to... 

Mercury(ll) is a very soft Lewis acid, which forms stable complexes preferentially with soft Lewis bases such as sulfur ligands. You should remember here that the major natural form of mercury is sulfides. Increasing the pH of the aqueous solution due to pollution or river water discharge to marine water leads to precipitation of HgO. We know that HgO has finite solubility in water, and the solution may be described in terms of mercury(II) hydroxide as the following reactions (2)-(4)... 

Despite this background reaction, the reaction rate is substantially accelerated by Lewis bases, which provides an opportunity for developing an asymmetric variant. The required enoxytrichlorosilanes 21.97 can be generated in various ways, e.g., from the corresponding trimethylsilyl enol ethers on reaction with silicon tetrachloride, catalysed by mercury(ii) acetate from carbonyl compounds or trimethylsilyl enol ethers on treatment with trichlorosilyl triflate from cx-chloroketones on reaction with trichlorosilane and triethylamine or from the corresponding tributyl-stannyl enol ethers, etc. ... 

It is clear from the preceding section that alkenes react as Lewis bases with polarizable dihalogens and with a classical Lewis acid such as borane. Can similar reactions occur with other Lewis acids The answer is yes, and an important class of Lewis acid is composed of mercury(II) compounds. 

The poisoning by heavy metal ions seems to result from their complexing of Lewis base positions on nucleic acids, enzymes, etc., which are then prevented from their normal operation. EDTA treatment is useful for the cases mentioned above. However, mercury ions in the body must be present as HgCl2, and other complexes which are not favorable for EDTA reaction. For example, take the case of HgCl J,... 

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