In a recent experimental work the Ir complex [Ir(cod)(py)(PCy3)](PF6) (that is, Crabtree’s catalyst) has been shown to catalyze the CH arylation of electron-rich heteroarenes with iodoarenes using Ag2CO3 as base. For this process, an electrophilic metalation mechanism, (SEAr) has been proposed as operative mechanism rather than the concerted metalation–deprotonation (CMD) mechanism, widely implicated in Pd-catalyzed arylation reactions. Herein we have investigated the CH activation step for several (hetero)arenes catalyzed by a IrIII catalyst and compared the data obtained with the results for the PdII-catalyzed CH bond activation. The calculations demonstrate that, similar to PdII-catalyzed reactions, the IrIII-catalyzed direct CH arylation occurs through the CMD pathway which accounts for the experimentally observed regioselectivity. The transition states for IrIII-catalyzed direct CH arylation feature stronger metal–C(arene) interactions than those for PdII-catalyzed CH arylation. The calculations also demonstrate that ligands with low trans effect may decrease the activation barrier of the CH bond cleavage.