(Figure , R 1 = aryl group). Conversely, cyclic(aryl)amino carbenes (36, 38 and 41, class I), cyclic(alkyl)amino carbenes (12, 15, 16 and 22, class Q) and their class K (R 1 = alkyl group) acyclic counterpart 5, and the monosubstituted carbene 17 (Figure , R 1 = H), among others, fit well within this correlation. The HOMO energy of a carbene is frequently used in the literature to assess its σ-donation ability. For the 124 structures studied, it is important to note that while the HOMO typically corresponds to the σ-type lone pair on the carbene carbon, this is not true for 18 structures.