Carbon—arsenic bonds

Quinn JP, G McMullan (1995) Carbon-arsenic bond cleavage by a newly isolated Gram-negative bacterium strain ASV2. Microbiology (UK) 141 721-727. 

Another synthesis of the carbon—arsenic bond is also most useful in making arsonomethyl compounds. The problems mentioned earlier... 

The overlap of carbon p orbitals with arsenic d orbitals is less effective than with the d orbitals of phosphorus, and so the covalent canonical structure is expected to make less of a contribution to the hybrid structure. This has been confirmed in an X-ray study of 2-acetyl-3,4,5-triphenylcyclopentadienetriphenylarsorane.6 Yamamoto and Schmidbaur7 found (13CNMR) that the bonding in arsenic ylides was probably sp3 (cf. phosphorus, which changes from sp3—>sp2), resulting in arsenic pseudotetrahedral geometry (cf. phosphorus ylides, which are planar). 

The development of arsenic derivatives for medicinal purposes bridges inorganic and organic chemistry as well as older and modern pharmaceutical practices. Although these compounds do contain at least one arsenic atom (a metalloid), they are still considered to be organic molecules. The highly covalent carbon-arsenic bond makes these organometal-lic compounds stable and easy to isolate. 

A particularly interesting class of S02-bridged polynuclear complexes is exemplified by the A-frame complex [PdCl(dpm)]2(S02), which results from addition of SO2 to the metal-metal bonded dimer [PdCl(dpm)]2 (Fig. 31). A unique feature of this complex is the reversibility of the SO2 binding across the metal-metal bond. Carbon monoxide effects a similar reaction with [PdQ(dpm)]2. X-ray photoelectron spectra have been reported for the SO2 complex and its arsenic analogue [PdQ(dam)]2(S02). Insertion... 

We have essentially exhausted all of the directly measured enthalpies of formation of compounds containing carbon-arsenic, -antimony and -bismuth bonds. However, let us now make use of other thermochemical data and see what can be derived using some plausible estimates. And barring that, let us see what new enthalpies of formation would become available if only some new measurement were made. 

Among the compounds containing six carbon-arsenic bonds only the spirocyclic compound 36 is known (equation 283 ). 

As Ip increases above values as low as 8.5, and we enter the upper-left portion of Fig. 3.4, the bonding between core cation and associated oxygen or hydroxyl is even stronger and largely covalent. The result is the formation of oxyanionic species such as silicate, selenate, borate, carbonate, arsenate, and sulfate, which, because of their relatively low charge densities as oxyanions, form rather weak bonds with cations and are soluble. 

There are two remaining systems for enzymatic changes in arsenicals, the periplasmic reduction of arsenate to arsenite as part of an oxyanion-coupled anaerobic respiration (16,68) (Chapter 13) and the coupled cleavage of carbon-arsenic bonds with oxidation to arsenate (7). These systems appear to be of major environmental concern in arsenic-containing settings, but they have not been approached by molecular genetics as yet. 

Bonds to Carbon.—Arsenic(ja) Compounds. Organoarsenic chemistry for 1971 has been reviewed. Methyldichloroarsine reacts with diazomethane to give the chloromethyl derivatives MeAs(CH2Cl)Cl and MeAs(CH2Cl)2, and the action of nucleophiles on these compounds is described. Ketoxime esters of benzenearsonous acid, PhAs(ON CR R )2, can be obtained by the reaction of PhAsO with ketoximes, while an alternative route from PhAsCl2 is also described. 

Edmonds, J.S. (2000) Diastereoisomers of an arsenomethionine -based structure from Sargassum lacerifolium the formation of the arsenic-carbon bond in arsenic-containing natural product. Bioorg. Med. Chem. Lett., 10, 1105-1108. 

Cobalt compounds have been in use for centuries, notably as pigments ( cobalt blue ) in glass and porcelain (a double silicate of cobalt and potassium) the metal itself has been produced on an industrial scale only during the twentieth century. Cobalt is relatively uncommon but widely distributed it occurs biologically in vitamin B12 (a complex of cobalt(III) in which the cobalt is bonded octahedrally to nitrogen atoms and the carbon atom of a CN group). In its ores, it is usually in combination with sulphur or arsenic, and other metals, notably copper and silver, are often present. Extraction is carried out by a process essentially similar to that used for iron, but is complicate because of the need to remove arsenic and other metals. 

---