The isolation and characterization of thermally stable groups R. Dimerizations of these electron-sextet molecules give rise to the double-bond systems of the digermenes, diaminocarbenes and diaminosilylenes has reawakened interest in the low-coordinated compounds of group-14 distannenes, and diplumbenes; the latter species have for a long time been considered as being incapable of existence. elements. The existence of these species is presumably due to interactions between the free electron pairs on the The properties of these molecules are compared with the results of quantum chemical calculations. A separate section nitrogen atoms and the carbene carbon atom or the silicon atom. The present review is mainly concerned with the is devoted to the isomers of Si 4 R 6 , which include novel tetrasilacyclobutenes and the diene analogues, the heavier analogues R 2 Ge:, R 2 Sn:, and R 2 Pb:, systems without intramolecular donor stabilization that owe their existence tetrasilabuta-1,3-dienes. principally to steric shielding by the voluminous alkyl or aryl MICROREVIEWS: This feature introduces the readers to the authors' research through a concise overview of the selected topic. Reference to important work from others in the field is included.