The first experimental observation of the NiH2.+ ion is described. Concerted hydrogen rearrangement upon mild collisional activation of the nickelocene molecular ion in either ion/surface or gas-phase ion/molecule collisions yields NiH2.+. Changes in the intensity of NiH2.+ with variation of collision and electron impact energies suggest that the mechanism involves the excitation of a low-energy vibrational mode of the molecular ion, resulting in cyclopentadienyl ring fusion to form C10H8 with the simultaneous transfer of two hydrogen atoms to Ni-.+. Attempts at the formation of NiH2.+ by direct insertion of Ni-.+ into hydrogen gas or by dehydrogenation of an alkane were unsuccessful; however, oxidative addition of H-2 to NiCp(.+) was observed. Gas-phase reactions between NiCp(n)(+) (n = 0-2) and 1,3- or 1,4-cyclohexadiene (CHD) show the production of ionic Ni complexes which involve dehydrogenation processes. These products include Ni(C6H6)(.+) NiCpH(2)(.+), and polyligated complexes with benzene, CHD, and cyclopentadiene. These dehydrogenation reactions suggest that the dihydride might be important in the hydrogenation/dehydrogenation chemistry of nickel.