Nickel(II) heterocyclic carbene complexes of the formula NiI2(carbene)(2) (carbene = 3-methyl-1-propylimidazolin-2-ylidene (1), 1-butyl-3-methylimidazolin-2-ylidene (2), 1-isopropyl-3-methylimidazolin-2-ylidene (3), 1,3-diisopropylimidazolin-2-ylidene (4)) have been prepared and fully characterized by both spectroscopic methods and single-crystal X-ray structure analysis. All complexes have "square-planar" nickel environments, with the donor atoms pairwise trans, all obligate, except for one of the two independent molecular types found in 2, in which, with different substituents at the nitrogen atoms, the two rotamers are observed as independent molecules in the one crystal. Complexes 1-4 were tested as catalysts for propene and 1-butene dimerization in both toluene and an imidazolium-chloroaluminateionic liquid. While the complexes are inactive to sparingly active in toluene, highly active catalysts are produced in the ionic liquid. In all cases the complexes are more active toward 1-butene dimerization than is NiCl2(PCy3)(2) in the ionic liquid. The results are interpreted in terms of catalyst stabilization by the imidazolium-based ionic liquid. In toluene the complexes decompose rapidly via reductive elimination of imidazolium cations, while in the ionic liquid no decomposition is observed. While reductive elimination of imidazolium is possible in the ionic liquid, a rapid reoxidation via addition of the solvent imidazolium cation seems possible and may prevent the formation of Ni-0 deposits associated with catalyst deactivation.