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5 Compounds of Group 14 (Ge, Sn, Pb)

DOI: 10.1055/sos-SD-005-00001

Moloney, M. G.; Thomas, E. J.Science of Synthesis, (200351.

General Introduction

The organometallic chemistry of the main group IV metals germanium, tin, and lead has been dominated by the widespread commercial exploitation of the latter two for antifungal agents (tributyltin oxide) and antiknock agents (tetraethyllead). Apart from its use as a doping agent in semiconductors, germanium has not found a similar bulk organometallic application, principally for reasons of cost. However, the long-term use of tin and lead has resulted in environmental contamination, and as a result of these legitimate environmental concerns, the use of these metals is increasingly restricted. For the synthetic chemist, however, organometallic compounds derived from all three of these metals are both readily accessible and synthetically valuable. Organometallic derivatives of germanium, tin, and lead are generally derived from the +4 oxidation state and are quadrivalent, but divalent germylenes, stannylenes, and plumbylenes are also known. Electronegativity decreases (Ge, 2.02; Sn, 1.72; Pb, 1.55) and atomic size increases (for example, for the +4 oxidation state, ionic radii are: Ge, 53pm; Sn, 71pm; Pb, 78pm) down the group, as expected, and organometallic derivatives from each of these metals participate in both single-electron (radical) and two-electron (ionic) processes. Their order of reactivity increases down the group, reflecting the stronger nature of GeH, GeO, GeCl, and GeC bonds relative to the same bonds with tin and lead, respectively, and this gradation of reactivity has been exploited in synthesis. Further control of reactivity of these organometallic derivatives is achievable by careful manipulation of ligands on the metal, in particular making use of steric effects. Germanium, tin, and lead all exhibit the ability to expand their coordination environment (so-called "hypervalency") and this has been turned to synthetic advantage particularly for organotin compounds. The lower reactivity of organogermanium compounds has recently been put to good effect in the development of selective reducing agents and in combinatorial chemistry applications, but organolead compounds, with a few exceptions, have not found widespread use. All three organometallics display the so-called "β-effect", in which a carbocationic center β to the metal is stabilized; this is responsible for some very useful synthetic reactivity. Indeed, these organometallic compounds exhibit considerable synthetic versatility, and they find application ranging from reducing agents to coupling partners for palladium-mediated reactions and many other transmetalation processes. However, all these main group organometallic derivatives should be handled with care, due to their known or potential toxicity and high fat solubility; such concerns are mitigated to some extent by the fact that these compounds are relatively nonvolatile, although methylstannanes are exceptional in this regard.

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