Aromaticity of an Unsaturated N‑Heterocyclic Stannylene (HCRN)2SnII As Studied by Optical Spectra and Quantum Chemistry. Comparison in the Series (HCRN)2EII, E = C, Si, Ge, Sn (R = t‑Bu or Dip)

The degree of π-electron delocalization in a series of unsaturated N-heterocyclic compounds containing divalent group-14 atoms, (HCRN)2EII (E = C, Si, Ge, Sn), has been estimated by applying different experimental and theoretical criteria (vibrational and UV–vis spectroscopy, nucleus-independent chemical shifts, and isomerization stabilization energies). All of the methods used confirmed the aromaticity of these heterocyclic molecules involving six π electrons, and most of the methods indicated that the aromaticity increases in going from the silylene to the stannylene.