5 Carbon, silicon, germanium, tin and lead

Abstract

A number of review articles relevant to this chapter have appeared. The area of carbon-rich ligands has been the subject of two timely publications, a special issue of the Journal of Organometallic Chemistry 1 on carbon-rich organometallic complexes and a complementary review on cumulenylidenes. 2 The catalytic applications of metal vinylidenes 3 and the occurrence of planar carbon in organometallic complexes of group 4 and 5 elements 4 have also been reviewed. For silicon, reviews have been in the area of the formation of monolayers 5 and the chemical processing of silicon wafers. 6 A review discussing the role of p-block elements, largely tin, in determining the structure of heterometallic complexes has been published. 7 Despite the discovery of fullerenes, the newest allotrope of carbon, much interest remains in one of the older examples, diamond. The reaction of carbon tetrachloride with four equivalents of sodium metal at 973 K in the presence of a Co/Ni catalyst yields C dia . The product, identi¢ed by Raman spectroscopy, is formed as a powder in a low (ca. 2%) yield, but nevertheless this is amongst the lowest energy routes to industrial diamonds yet known. 8 The same product can be prepared using conditions designed to mimic those found on Uranus and Neptune in which methane is heated to (2^3) Â 10 3 K at pressures of 10^50 gigapascals, whereupon the methane breaks down to form diamond and polymeric hydrocarbons. 9 A convincing argument has been presented for the formation of such a dense solid from a light gaseous phase contributing to the internal heat, magnetic ¢eld and luminosity of a celestial body, and hence the possible signi¢cance of this process in planet formation. A report of the synthesis and identi¢cation of C 6 CH 2 as :C( C C) 2 C CH 2 by removal of the charge from the C 6 CH 2 À. radical anion during neutralisationreionisation mass spectroscopy has been published and related to the cumulenes found in circumstellar dust. 10 Cumulene complexes of transition metals continue to attract much attention. Although synthetic protocols are well established, some new approaches have appeared outlined in Scheme 1. 11,12 The last compound shown is a very rare example of a linear ten atom chain C 8 Rh 2 . Cumulenes with extremely long carbon chains have also appeared, such as 3 which shows in the solid state a distinct twisting of the C 12 chain away from linear, 13 and 4 and 5 (Scheme 2). The crystal structure of 5 shows C^C bond lengths of 1.210^1.233 Ð for triple bonds