Cyclic polyenes of heavy group 14 elements: new generation ligands for transition-metal complexes.

Cyclic polyenes, such as benzene, cyclopentadienyl and cyclobutadiene, are widely used as key ligands for a variety of transition-metal complexes. The heavy versions of these compounds, in which the skeletal carbon atoms are fully (or partially) replaced with heavy group 14 elements (Si, Ge, Sn and Pb) were not synthetically accessible until quite recently. However, they are now readily available following the pioneering discoveries of derivatives of sila- and germabenzenes, sila- and germacyclopentadienide ions and tetrasila- and disiladigermacyclobutadiene dianions. Apart from their undoubted structural and synthetic interest, such organometallic compounds are particularly important as the precursors of novel ligands for new-generation transition-metal complexes, and this is covered in this critical review (124 references).