The activation of the CH bond of 1-phenylpyrazole (2) and 2-phenyl-2-oxazoline (3) by [Ru(OAc)2(p-cymene)] is an autocatalytic process catalyzed by the co-product HOAc. The reactions are indeed faster in the presence of acetic acid and water but slower in the presence of a base K2CO3. A reactivity order is established in the absence of additives: 2-phenylpyridine>2-phenyl-2-oxazoline>1-phenylpyrazole (at RT). The accelerating effect of added acetate ions reveals an intermolecular deprotonation after CH bond activation by a cationic RuII center (SE3 mechanism). The reactions of 1-phenylpyrazole and 2-phenyl-2-oxazoline first lead to the neutral cyclometalated complexes A2 and A3 ligated by one acetate. The latter dissociate to the cationic complexes B2+ and B3+, respectively, and acetate. A slow incorporation of one or two D atoms into 2, 3, and 2-phenylpyridine (1) was observed in the presence of deuterated acetic acid. The “reversibility” of the CH bond activation/deprotonation takes place from the cationic complexes Bn+ (n=1–3). They are also involved in oxidative additions to PhI, which are rate-determining and lead to the mono- and bis-phenylated products at high temperatures. A general mechanism is proposed for the arylation of arenes 1–3 catalyzed by [Ru(OAc)2(p-cymene)]. In contrast, the reaction of Pd(OAc)2 with 2-phenylpyridine (1), is much faster: Pd(OAc)2>[Ru(OAc)2(p-cymene)]. Since the kinetics is not affected by added acetates, the reaction proceeds through a CMD mechanism assisted by a ligated acetate (intramolecular process) and is irreversible. A bis-cyclometalated PdII^PdII dimer D′1 is formed whose bielectronic electrochemical oxidation leads to a [PdIII^PdIII]2+ dimer, in agreement with the result of a reported chemical oxidation used in arene functionalizations catalyzed by Pd(OAc)2.