Electronic Structural Trends in Divalent Carbon Compounds

This work aims to analyze and compare the intrinsic electronic densities in a series of neutral and anionic divalent carbon-donor derivatives. The sigma-lone pair at the divalent carbon is the HOMO of these species. Structural factors have been identified that influence its energy, which is a measure of the sigma-basicity. The pi-electronic structure has been described as a function of the pi-population. Our results show that no straightforward structural criteria correlate with the pi-electronic distribution. However, the pi-population, as well as the pi-acidity and pi-basicity, are related to the pi-MOs. In all cases, these pi-MOs can be qualitatively obtained on the basis of those of the protonated analogues by simply increasing the energy of the p(pi) orbital at the divalent carbon atom compared to normal sp(2) carbon. Such an analysis allows a rationalization of the trends observed for the pi-electronic structure of these ligands. Notably, this explains the values of the pi-population at the divalent carbon center, which shows an increasing and continuous range from classical NHCs to mesoionic carbenes.