The discrepancy between HEP and 1 JC-H raises the question of which of theses two donation scales should be used to compare ligands of different classes. To address this question, we used the ETS-NOCV method, which allows decomposition of a ligand binding energy within a complex into its different contributions, including -donation (noted E ETS-NOCV ). To avoid large computations, we calculated the -donation component of the C-H bond in the azolium cation where the carbene is coordinated to the proton as the Lewis acid. The ETS-NOCV calculations were performed for the 42 molecules with available HEP data. The comparison between E ETS-NOCV and 1 JC-H shows reasonable agreement (Figure ; R 2 = 0.86). However, compounds 8H and 123H, belonging to classes K and J, respectively, are far from the correlation line. Excluding these outliers leads to a coefficient of determination of 0.91 (Figure ). This indicates that the ETS-NOCV and 1 JC-H approaches provide comparable estimates of the -donor Excluding the 4 main outliers 8, 18, 24, and 31 from Figure , or all compounds of classes K, E, and D has a minimal effect on the correlation, with a slight increase of R 2 to 0.91 and 0.92, respectively (Figure ). On the other hand, comparing the -donation from ETS-NOCV calculations and HEP values for the 42 compounds shows a lack of correlation (Figure ; R 2 = 0.08). Exclusion of the previous outliers (8, 18, 24, and 31) or all compounds of classes K, E, and D, significantly improves the correlation (Figure ; R 2 = 0.47 and 0.84, respectively), which however remains lower than that obtained for 1 JC-H. The deceptive result for HEP may be related to several factors. It is known that the paramagnetic contribution to the NMR shift depends on various factors, such as the nature of the magnetically coupled orbitals, their spatial arrangement, and the relevant carbene-centered orbital energies. Thus, steric effects or anisotropy effects can have a substantial influence, leading to HEP values that are problematic. The presence of compounds 8, 18, 24, and 31 significantly weakens the correlations with HEP, yet does not notably affect the correlation between JC-H and E ETS-NOCV . This suggests that the HEP values for these four compounds may be unreliable. The choice of protonated complexes for the ETS-NOCV calculations could also contribute to the disappointing results in the comparison with HEP. The organometallic fragment [PdBr2( i Pr2-bimy)] used for HEP measurement is significantly larger than a proton, and carbenes with flexible substituents, such as benzyl groups, may adopt different conformations in the measurements of HEP and E ETS-NOCV . However, studies on different conformers (see SI) do not improve the agreement between HEP measurements and 1 JC-H or E ETS-NOCV . Finally, it is also possible that E ETS-NOCV may follow different trends when the carbene is bound to a proton or a metal fragment, although this aspect is beyond the scope of this study. Overall, these comparisons between JC-H, HEP, and E ETS-NOCV suggest that 1 JC-H is a more reliable indicator of σdonating ability than HEP when comparing carbenes of different classes. However, for specific carbenes, such as acyclic alkylamino-(K) and aminooxy-carbene (J), even