Pure and Applied Chemistry

Abstract: The dianions of tetraphenylsilole and tetraphenylgermole behave as aromatic species, as shown by the equalization of C-C bond lengths in these rings and by nucleus-independent chemical shift (NICS) calculations. Similarly, the dianions of 9-silafluorene and 9-germafluorene are aromatic. In the latter species, the heteroatom five-membered rings are more aromatic than the attached benzene rings. Also to be recognized as aromatically stabilized compounds are the stable unsaturated diiminocarbenes, diiminosilylenes, and diiminogermylenes. Evidence for the aromatic delocalization in these molecules comes from NMR, NICS calculations, Raman spectroscopy, isodesmic molecular orbital (MO) calculations, and calculated isomerization stabilization energies. From the presently available evidence, the aromaticity appears to alternate, with C > Ge > Si, going down group 14.