Dimethylmercury

Dimethylmercury ((CH3)2Hg) is an extremely toxic organomercury compound. A highly volatile, reactive, flammable, and colorless liquid, dimethylmercury is one of the strongest known neurotoxins, with a quantity of less than 0.1 mL capable of inducing severe mercury poisoning resulting in death, and is easily absorbed through the skin. Dimethylmercury is capable of permeating many materials, including plastic and rubber compounds. It has a slightly sweet odor, although inhaling enough of the chemical to notice this would be hazardous.
The acute toxicity of the compound was demonstrated by the 1997 death of heavy metal chemist Karen Wetterhahn, who died 10 months after a single exposure on August 14th, 1996 of only a few drops permeated through her disposable latex gloves which were at the time mistakenly believed to offer adequate protection.
Synthesis, structure, and reactions.
The compound was one of the earliest organometallics reported, reflecting its considerable stability. The compound was first prepared by George Buckton in 1857 by a reaction of methylmercury iodide with potassium cyanide:
Later, Frankland discovered that it could be synthesized by treating sodium amalgam with methyl halides:
It can also be obtained by alkylation of mercuric chloride with methyllithium:
The molecule adopts a linear structure with Hg–C bond lengths of 2.083 Å.
Reactivity and physical properties.
An unusual feature of this compound is its low reactivity towards proton sources, being stable to water and reacting with mineral acids at a significant rate only at elevated temperatures, whereas the corresponding organocadmium and organozinc compounds (and most metal alkyls in general) hydrolyze rapidly. The difference reflects the high electronegativity of Hg (Pauling EN = 2.00) low affinity of Hg(II) for oxygen ligands. The compound undergoes a redistribution reaction with mercuric chloride to give methylmercury chloride:
Whereas dimethylmercury is a volatile liquid, methylmercury chloride is a crystalline solid.
Use.
Dimethylmercury currently has few applications because of the risks involved. As with many methyl-organometallics, it is a methylating agent that can donate its methyl groups to an organic molecule; however, the development of less acutely toxic nucleophiles such as dimethylzinc and trimethylaluminium, and the subsequent introduction of Grignard reagents (organometallic halides), has essentially rendered this compound obsolete in organic chemistry. It was formerly studied for reactions in which the methylmercury cation was bonded to the target molecule, forming potent bactericides; however, the bioaccumulation and ultimate toxicity of methylmercury has largely led it to be abandoned for this purpose in favor of the less toxic diethylmercury and ethylmercury compounds, which perform a similar function without the bioaccumulation hazard.
In toxicology, it was formerly used as a reference toxin. It has also been used to calibrate NMR instruments for detection of mercury (δ 0 ppm for 199Hg NMR), although diethylmercury and less toxic mercury salts are now preferred.
Safety.
Dimethylmercury is extremely toxic and dangerous to handle. As early as 1865, two workers in the laboratory of Frankland died after exhibiting progressive neurological symptoms following accidental exposure to the compound. Absorption of doses as low as 0.1 mL can result in severe mercury poisoning. The risks are enhanced because of the high vapor pressure of the liquid.
Permeation tests showed that several types of disposable latex or polyvinyl chloride gloves (typically, about 0.1 mm thick), commonly used in most laboratories and clinical settings, had high and maximal rates of permeation by dimethylmercury within 15 seconds. The American Occupational Safety and Health Administration advises handling dimethylmercury with highly resistant laminated gloves with an additional pair of abrasion-resistant gloves worn over the laminate pair, and also recommends using a face shield and working in a fume hood.
Dimethylmercury is metabolized after several days to methylmercury. Methylmercury crosses the blood–brain barrier easily, probably owing to formation of a complex with cysteine. It is eliminated from the organism slowly, and therefore has a tendency to bioaccumulate. The symptoms of poisoning may be delayed by months, resulting in cases in which a diagnosis is ultimately discovered, but only at the point in which it is too late for an effective treatment regimen to be successful.
The toxicity of dimethylmercury was highlighted with the death of Karen Wetterhahn, a professor of chemistry at Dartmouth College, in 1997. Professor Wetterhahn specialized in heavy metal poisoning. After she spilled a few drops of this compound on her latex glove, the barrier was compromised, and within seconds it had permeated her gloves and was absorbed into her skin. It circulated through her body and accumulated in her brain, resulting in her death ten months later. This accident is a common toxicology case-study and directly resulted in improved safety procedures for chemical-protection clothing and fume hood use, which are now called for when any exposure to such severely toxic and/or highly penetrative substances is possible (e.g., in chemical munitions stockpiles and decontamination facilities).