Density functional theory calculations have been performed on intramolecular C(aryl)-X bond activation reactions in model N-heterocyclic carbene (NHC) complexes of the type Ru(NHC)(PH3)2(CO), where NHC = 1-(C6H4-2-X)imidazol-2-ylidene (I(o-C 6H4X), X = H, CH3, F, OH, NH2, OCH3, and CF3). In all cases C(aryl)-X activation is found to be thermodynamically favored, and the largest barrier to reaction is computed to be +21.3 kcal/mol when X = CH3. As C(aryl)-CH3 bond activation has been observed experimentally for a Ru-NHC complex (Jazzar, R. F. R.; Macgregor, S. A.; Mahon, M. F.; Richards, S. P.; Whittlesey, M. K. J. Am. Chem. Soc. 2002, 124, 4944), this suggests that a wide range of heteroatom-substituted N-aryl NHC ligands may be susceptible to intramolecular bond activation and potential ligand degradation. The computed exothermicity of C(aryl)-X activation follows the trend X = NH2 < CH3 < H < OH ˜ OCH3 < CF3 < F, while the barriers vary as X = H < F < OH ˜ OCH3 < CF3 < NH2 < CH3. Both series reflect the promotion of C(aryl)-X activation by the formation of stronger Ru-X bonds in the product. However, the ability of heteroatom ligands to stabilize the Ru(O) reactants through chelation can disfavor C(aryl)-X cleavage and explains the low exothermicity and high barrier associated with C(aryl)-NH2 activation. For X = OCH3 C(aryl)-O bond activation was found to be favored kinetically over O-C(alkyl) activation, although the latter process yields an extremely stable aryloxide product. The arrangement of coligands around the metal can significantly affect C(aryl)-X bond activation, and when X is a p-donor, this process is promoted by a trans CO ligand. These insights suggest not only possible ways to control unwanted C(aryl)-X activation in heteroatom-subtituted N-aryl NHC ligands but also factors that may promote such reactions in catalytic processes where this step is desirable. © 2008 American Chemical Society.