The reaction of RUH2(H-2)(2)(PCy3)(2) (1) with L-X in pentane at room temperature yields new hydrido derivatives of ruthenium accommodating a stretched H-H bond, namely RuH(H-2)(L-X)(PCy3)(2) (L-X = C5H4N-O, 2; L-X = C5H4N-NH, 3). NMR studies show that hydrogen bond donors (substituted phenols, hexafluoro-2- propanol. etc.) interact with the hydrides in the case of 2, whereas for 3 an equilibrium with the cation [RuH-(H-2) (py-NH2)(PCy3)2](+) is attained. The latter species has been isolated in the form of the [B(C6F5)(4)] salt, 4, independently prepared by addition of (PhNMe2H)[B(C6F5)(4)] to 3. These phenomena explain the difference of reactivity with olefins between 2 and 3 in nonpolar media or in the presence of alcohols.