Electronic structures of main-group carbene analogues.

The electronic structures of 15 group 13-16 carbene analogues are analyzed using various quantum chemical methods and compared to the data obtained for the parent N-heterocyclic carbene (NHC), imidazol-2-ylidene. The results of this study present a uniform analysis of the similarities and differences in the electronic structures of p-block main-group carbene analogues. Though all systems are formally isovalent, the theoretical analyses unambiguously indicate that their electronic structures run the gamut from C=C localized (group 13) to C=N localized (group 16) via intermediate, more delocalized, systems. In particular, neither the stibenium ion nor any of the chalcogenium dications is a direct analogue of imidazol-2-ylidene as they all contain two lone pairs of electrons around the divalent main-group center, instead of the expected one. The reason behind the gradual change in the electronic structure of main-group analogues of imidazol-2-ylidene was traced to the total charge of the systems, which changes from anionic to dicationic when moving from left to right in the periodic table. Results from theoretical analyses of aromaticity show that all group 13-16 analogues of imidazol-2-ylidene display some degree of aromatic character. The heavier group 13 anions benefit the least from pi-electron delocalization, whereas the cationic group 15 systems are on par with the parent carbon system and display only slightly less aromatic character than cyclopentadienide, a true 6pi-electron aromatic species. The sigma-donor and pi-acceptor ability of the different main-group carbene analogues are also evaluated.