Ni(H)(X)(IMes)(2) was prepared by the addition of HX to Ni(IMes)(2) (X = Cl, Br; IMes = 1,3-dimesitylimidazol-2-ylidene). Ni(H)(Cl)-(IMes)(2) (1) was isolated from the reaction mixture of Ni(IMes)2 and 2 equiv of 2,6-lutidine center dot HCl. Ni(H)(Br)(IMes)(2) was prepared in a similar way. Although treatment of Ni(IMes)(2) with a HCl center dot dioxane solution gives rise to a mixture of 1, NiCl2(IMes)2(,) and NiCl(IMes)(2), 1 was not isolable from the mixture. All three complexes cocrystallized. Photolysis of these nickel hydrides activates their Ni H bonds by populating Ni-H sigma(star) orbitals, which results in the formation of H-2. Treatment of 1 with HCl center dot dioxane gives rise to H-2 and NiCl2(IMes)(2).