and HEP values, illustrating that these two parameters measure a comparable effect for the substituents. However, caution is required when extending this correlation to different classes of NHC. Figure shows a general trend, mainly disturbed by the compounds of classes K, E, and to a lesser extent D. Interestingly, the reliability of the HEP values of the most outliers (8, 18, 24, 31 ) has been questioned due to anisotropic effect. The correlation between HEP and 1 JC-H remains very modest (R 2 =0.58) when these four ligands were excluded as outliers and improves noticeably (R 2 =0.84) when all compounds of classes K, E, and D were excluded (Figure ).