The protonation with Brønsted acids HB (HBF4 and CF 3CO2H) of [RuH2(arene)(PPh2Het)] derivatives (PPh2Het = dpim, Het = 2-N-methylimidazolyl; PPh 2Het = PPh2py, Het = 2-pyridyl) that contain uncoordinated N atoms has been analyzed experimentally by NMR spectroscopy and through computational studies. Initially, at low temperature, the uncoordinated N atoms of the phosphine are protonated and a proton-hydride exchange is observed by 1H NMR spectroscopy. The proton transfer leads to the corresponding cationic trihydride intermediates, which exhibit a dual character of classical and nonclassical hydrides, with the nonclassical species being more stable, especially when Het = 2-pyridyl. In fact, the release of H2 and the formation of the respective monohydride complexes [RuH(arene) (κ2-N,P-PPh2Het)]B was observed at room temperature. The participation of the uncoordinated N center in the proton transfer process in the stabilization of RuH(H2)+ with respect to RuH 3+ species and in the hydride-dihydrogen exchange (cis effect) are discussed. Calculations on the complex [RuH3(p-cymene) (PPh3)]+ have also been carried out for the sake of comparison. A dual character was also found, but in this case the classical species is more stable. H/D exchange of the hydride ligands of the dihydride complexes, using CD3OD as the deuterium source, has been studied. The very rapid deuterium labeling of H2, catalyzed by the aforementioned dihydrides, has been observed. The known compound [RuH2(p-cymene) (PPh3)] is also active in this labeling process, and the possible mechanism for both the H/D exchange and the deuterium labeling of H2 is discussed in light of theoretical studies. © 2012 American Chemical Society.