Carbon-carbon bond activation in cationic 6-endo-methyl-eta(5)-cyclohexadienyl and 6-exo-methyl-eta(5)-cyclohexadienyl ruthenium hydride complexes has been investigated. Contrary to expectations, it is the 6-exo-methyl complex and not the stereoisomeric 6-endo-methyl complex that undergoes selective carbon-carbon bond activation under exceptionally mild conditions, quantitatively converting the 6-exo-methyl substituent and the hydride ligand to methane. The mechanism of the activation reaction involves dissociation of protic acid from the agostic starting complex by reaction with a weak base (typically water), followed by protolytic activation of the alkyl group. with "back-side" assistance from the nucleophilic metal center. Under the same conditions, the corresponding 6-endo-methyl isomer undergoes selective dehydrogenation rather than demethylation, despite the proximity of the Endo-methyl substituent to the metal center. For both exo and endo isomers, the cationic ruthenium hydride intermediates were determined by spectroscopic analysis to adopt fluxional agostic structures. The agostic complexes an kinetically stable at room temperature under rigorously anhydrous conditions but convert quantitatively to cationic eta(6)-arene products in the presence of a Bronsted base. The rates of both carbon-carbon bond activation and dehydrogenation are dependent on the identity and concentration of the base and suppressed in the presence of excess acid. The protolytic mechanism for carbon-carbon bond activation is supported by deuterium-labeling studies and by the reactivity of the neutral complexes toward Lewis acids and one-electron oxidants. This mechanism is shown to be relevant to carbon-carbon bond activation reactions observed in less-substituted 6-exo-methyl-eta(5)-cyclohexadienyl complexes and in a steroid-derived 6,6-disubstituted-eta(5)-cyclohexadienyl complex, representative of previously reported cases of dealkylative ligand aromatization. The low kinetic barrier for the protolytic dealkylation mechanism is contrasted to the comparatively high activation barriers reported for carbon-carbon bond activation reactions that occur in structurally related systems that cannot access a protolytic pathway. This investigation provides a consistent basis for rationalizing this potentially important but poorly understood class of metal-mediated reactions.