A dinuclear Co complex with bis(pyridyl)-pyrazolato (bpp(-)) and terpyridine (trpy) ligands, [Co-2(III)(trpy)(2)(mu-bpp)(OH)(OH2)](4+) (1(4+)), undergoes three-electron reduction by cobaltocene in acetonitrile to produce 1(+), which is in the protonation equilibrium with the (CoCoIII)-Co-II-hydride complex, and the further protonation of the hydride by trifluoroacetic acid yields hydrogen quantitatively. The kinetic study together with the detection of the (CoCoIII)-Co-II-hydride complex revealed the mechanism of the hydrogen production by the reaction of 1(+) with trifluoroacetic acid.