Therefore, the energy of the carbene lone pair (ELP), corresponding to the HOMO energy in most cases but to the HOMO-1 or HOMO-2 energy in others, was compared to JC-H and ΔEσ ETS- NOCV values. Contrary to what is suggested in the literature, there is a weak correlation between ELP and 1 JC-H (Figure ). While charged molecules are clear outliers, their exclusion only modestly improves the correlation (R 2 = 0.70). Furthermore, restricting the analysis to specific classes of compounds does not significantly enhance the correlation. A similar result is observed when comparing ELP to ΔEσ ETS-NOCV (Figure ), confirming that σ-donation is poorly estimated by ELP. Finally, we investigated whether carbene properties could be determined using proton or carbon chemical shifts instead of the for example, to assess the coordination properties of carbene ligands using the isotropic chemical shielding of the acidic proton. The calculated data, which do not suffer from counterion effects, can serve as a pertinent test. Unfortunately, the comparison is clear: all comparisons between 1 H or C chemical shifts and the other parameters studied previously show a lack of correlation (Figure ).