Nickel-mediated formation of H-2 by protonation of Ni(I) has been established and the kinetics of the process investigated. The diamagnetic complex [Ni-II(psnet)](BF4)(2) was prepared and reduced to [Ni-I(psnet)](BF4) with NaBH4 in THF (psnet = bis(5-(diphenylphosphino)-3-thiapentanyl)amine). Both complexes were structurally characterized by X-ray diffraction. [Ni(psnet)](1+) was demonstrated to be an authentic Ni(I) complex with a ...(d(z2))(1) ground state. Under appropriate conditions, [Ni(psnet)](+) reacts with acids in nonaqueous media to give near-quantitative yields of H-2 according to the stoichiometry Ni-I + H+ --> Ni-II + 1/2H(2). Dihydrogen production was demonstrated to be directly related to Ni(I) oxidation. The reaction system [Ni(psnet)](+)/HCl/DMF, which gives H-2 yields of greater than or similar to 90%, was subjected to a kinetics analysis. The overall reaction [Ni(psnet)](+) + HCl --> [Ni(psnet)Cl](+) + 1/2H(2) proceeds by two parallel pathways dependent on chloride concentration. Addition of Bu(4)NCl accelerates the reaction, whereas (Bu(4)N)(PF6) decreases the rate. A two-term rate law is presented which includes contributions from both pathways, whose common initial step is protonation of Ni(I). Path A (low chloride concentration) involves the formation and collapse of nickel hydride chloride ion pairs; the rate-determining step is the minimal reaction 2Ni(III) - H- --> H-2 + 2Ni(II). Path B (high chloride concentration) includes as the rate-limiting step collapse of a nickel hydride dichloride ion pair followed by the bimolecular reaction of two Ni-III - H- intermediates or reduction to Ni-II - H- by Ni-I followed by protonation of the hydride. The relation of these results to the reactions of hydrogenase enzymes is considered.