Selenium metal-carbon bond

Organometallic compounds are those in which there is a metal-carbon bond. According to this definition, in the case of transition metals, this group of compounds includes not only metal carbonyls, olefin complexes, cyclopentadienyl, and other 7r-complexes, but also cyanide and fulminate compounds. Certain difficulties arise in defining the metal of the main group elements. Usually, organometallic compounds are comprised not only of compounds of typical metals, but also of metalloids such as boron, silicon, phosphorus, arsenic, selenium, etc. In compounds of metals as well as in those of metalloids, the bond is generally polarized as follows C. Consequently, the... 

Metallaheterocycles with the metal atom capable of supporting a carbon-metal double bond, and with oxygen, nitrogen, sulfur, or selenium as the second heteroatom are the only structures capable of full conjugation. These compounds show furan-like aromaticity with the heteroatom /i-electrons participating. No examples of this type of ring system were uncovered, undoubtedly due to the stability of the metalla-carbon double bonds required for their formation. 

The osmium carbyne complex 115 reacts with elemental sulfur, selenium, and tellurium to afford the complexes 135 in which the element atoms "bridge the metal-carbon triple bond [Eq. (123)] (56). Complex 115 also reacts with transition metal Lewis acids such as AgCl or Cul to give dinuclear compounds with bridging carbyne ligands. Reaction with elemental chlorine results in addition across the metal-carbon triple bond to generate the chlorocarbene osmium complex 136 [Eq. (124)]. 

A few metal complexes of selenothioic and diselenoic acids have been reported. In a study on the reactivity of metal-carbon triple bond compounds toward chalcogens, it was found that molybdenum (52) and tungsten (53) carbyne complexes reacted with elemental selenium to yield the corresponding di-selenoate complexes 54 and 55, respectively (Eqs. 13 and 14) [18]. 

A final and unusual type of carbon-bonded acetylacetonate is that in which the metal atom forms a derivative at a terminal or 1-carbon. The only compounds in this case are three tellurium derivatives shown in Fig. 1G, H, I. These compounds were first prepared by Morgan and Drew (33) in the 1920 s, and on the basis of chemical evidence they correctly postulated the structure of 1G, but not of 1H or 11. Recent spectroscopic studies by Dewar et al. (34) have confirmed the structure of 1G and determined the structures illustrated in Fig. IH and II. It is interesting to note here that sulfur and selenium form bonds only to the 3-carbon atom. At present it is difficult to rationalize the loss of hydrogen for a terminal methyl group which is many orders of magnitude less reactive than the 3-protons. 

Rubber. The mbber industry consumes finely ground metallic selenium and Selenac (selenium diethyl dithiocarbamate, R. T. Vanderbilt). Both are used with natural mbber and styrene—butadiene mbber (SBR) to increase the rate of vulcanization and improve the aging and mechanical properties of sulfudess and low sulfur stocks. Selenac is also used as an accelerator in butyl mbber and as an activator for other types of accelerators, eg, thiazoles (see Rubber chemicals). Selenium compounds are useflil as antioxidants (qv), uv stabilizers, (qv), bonding agents, carbon black activators, and polymerization additives. Selenac improves the adhesion of polyester fibers to mbber. 

Furthermore, the strongly metallic character of selenium weakens the C-Se bond and thus favors reactions involving opening of the ring. The basicity of the three heterocycles is approximately in the same order, the nitrogen atom of selenazole and thiazole possessing much the same properties as the heteroatom of pyridine. Of the two carbon atoms ortho to nitrogen, that is, the 2-carbon and the 4-carbon, only the one in the 2-position is fairly active as a result of its interaction with selenium or sulfur. The 4- and 5-positions of thiazole and selenazole are more susceptible to electrophilic substitution than the 3- and 5-positions of pyridine. This is particularly true of the 5-position of selenazole. Thus it can be said that the 2- and 5-positions of the selenazoles and thiazoles... 

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