Treatment of cis-[W(N-2)(2)(PMe2Ph)(4)] (5) with an equilibrium mixture of trans-[RuCl(eta (2)-H-2)(dppp)(2)]X (3) with pK(a) = 4.4 and [RuCl(dppp)(2)]X (4) [X = PF6, BF4, or OTf; dppp = 1,3-bis(diphenylphosphino)propane] containing 10 equiv of the Ru atom based on tungsten in benzene-dichloroethane at 55 degreesC for 24 h under 1 atm of H-2 gave NH3 in 45-55% total yields based on tungsten, together with the formation of trans-[RuHCl(dppp)(2)] (6). Free NH3 in 9-16% yields was observed in the reaction mixture, and further NH3 in 36-45% yields was released after base distillation. Detailed studies on the reaction of 5 with numerous Ru(eta (2)-H-2) complexes showed that the yield of NH3 produced critically depended upon the pK(a) value of the employed Ru(eta (2)-H-2) complexes. When 5 was treated with 10 equiv of trans-[RuCl(eta (2)-H-2)(dppe)(2)]X (8) with pK(a) = 6.0 [X = PF6, BF4, or OTf; dppe = 1,2-bis(diphenylphosphino)ethane] under 1 atm of H-2, NH3 was formed in higher yields (up to 79% total yield) compared with the reaction with an equilibrium mixture of 3 and 4. If the pK(a) value of a Ru(eta (2)-H-2) complex was increased up to about 10, the yield of NH3 was remarkably decreased. In these reactions, heterolytic cleavage of H-2 seems to occur at the Ru center via nucleophilic attack of the coordinated N-2 on the coordinated H-2 where a proton (H+) is used for the protonation of the coordinated N-2 and a hydride (H-) remains at the Ru atom. Treatment of 5, trans-[W(N-2)(2)(PMePh2)(4)] (14), or trans-[M(N-2)(2)(dppe)(2)] [M = Mo (1), W (2)] with Ru(eta (2)-H-2) complexes at room temperature led to isolation of intermediate hydrazido(2-) complexes such as trans-[W(OTf)(NNH2)(PMe2Ph)(4)]OTf (19), trans-[W(OTf)(NNH2)(PMePh2)(4)]OTf (20), and trans-[WX(NNH2)(dppe)(2)](+) [X = OTf (15), F (16)]. The molecular structure of 19 was determined by X-ray analysis. Further ruthenium-assisted protonation of hydrazido(2-) intermediates such as 19 with H-2 at 55 degreesC was considered to result in the formation of NH3. concurrent with the generation of W(VI) species. All of the electrons required for the reduction of Na are provided by the zerovalent tungsten.