Arsenic bonds with Group 13 compounds

The magnetic criterion is particularly valuable because it provides a basis for differentiating sharply between essentially ionic and essentially electron-pair bonds Experimental data have as yet been obtained for only a few of the interesting compounds, but these indicate that oxides and fluorides of most metals are ionic. Electron-pair bonds are formed by most of the transition elements with sulfur, selenium, tellurium, phosphorus, arsenic and antimony, as in the sulfide minerals (pyrite, molybdenite, skutterudite, etc.). The halogens other than fluorine form electron-pair bonds with metals of the palladium and platinum groups and sometimes, but not always, with iron-group metals. 

A detailed study of the radiochemical reactions of phenylarsenic compounds has been published by Grossmann. Once again unable to effect isolation of all compounds, he was able, however, to get evidently reliable values for the sums of all compounds with one, two and three phenyl-arsenic bonds, respectively, as well as ionic arsenic and a further organic-soluble fraction which appeared to be a group of polymeric phenylarsenic compounds. Selected data from this work are given in Table 6. 

The methyl arsenic system compromises two well-defined species, the pentamer (MeAs)5, a yellow pyrophoric oil (m.p. 12°C) and the purple black double-chain polymer, [(MeAs)2] (m.p. 204 °C). A red solid that is possibly the linear chain compound (MeAs) has also been described. The polymers are formed when samples of the pentamer are exposed to impurities like arsenic halides that are able to react with the arsenic-arsenic bond and may act as end groups. The most widely used method for the preparation of larger amounts of (MeAs)j is the reduction of methylarsonic acid, MeAsO(OH)2, or its sodium salt with hypophosphoric acid " °. Samples prepared by this method, however, may contain between 5 and 15% of impurities. A method that minimizes impurities is the reaction of MeAsHj with dibenzylmercury (equation 3) °. 

This chapter is concerned with the solid state structures of arsenic, antimony and bismuth compounds, which in general contain at least one bond between carbon and the Group 15 element. In selecting material for discussion, I have been greatly aided by the availability of the Cambridge Crystallographic Data Base. The period under consideration covers effectively the years between 1981 and 1992. 

This article is concerned mainly with the simple halides of phosphorus, arsenic, antimony, and bismuth compounds that also contain other elements or groups will be considered only to a limited extent. Nitrogen compounds show special features associated with the absence in nitrogen of d electrons for bonding, and are not easily discussed together with compounds of the heavier elements they will therefore be omitted. Bismuth has metallic properties that differentiate its compounds from those of elements in the middle of the group the break in properties between the compounds of antimony and bismuth is not, however, so marked as that between compounds of nitrogen and phosphorus. 

The metals discussed in this chapter—lead, arsenic, mercury, and iron—frequently cause significant toxicity in humans. The toxicity profiles of metals differ, but most of their effects appear to result from interaction with sulfhydryl groups of enzymes and regulatory proteins. Chelators are organic compounds with two or more electronegative groups that can form stable covalent-coordinate bonds with cationic metal atoms. As emphasized in this chapter, these stable complexes can often be excreted readily, thus reducing the toxicity of the metal. 

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. 

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