Some recent advances in the chemistry of silicon and its homologues in low coordination states

This review summarises some recent results on synthesis, structures, and reactivity of the heavier analogues of carbenes, alkenes, and dienes. With the isolation of the first diplumbenes, the series R2E=ER2, E = C-Pb, has been completed. These double bond systems show enormous variations in the E=E bond lengths, E = Si-Pb, and the trans-bent angles. Furthermore, new families of multiple-bonded species have been discovered, such as a spiropentasiladiene, molecules with conjugated Si=Si, Ge=Ge, and Ge=C double bonds, and a diplumbylene RPbPbR. However, no stable compounds featuring homonuclear triple bonding between heavier Group 14 elements are known. Theoretical insights on the nature of such triple bonds as well as first attempts to achieve this target are discussed. (C) 2002 Elsevier Science B.V. All rights reserved.