N-heterocyclic carbene ligands IMes (1), SIMes (2), IPr (3), SIPr (4), and lCy (5) react with Ni(CO)(4) to give the saturated tricarbonyl complexes Ni(CO)(3)(IMes) (8), Ni(CO)(3)(SIMes) (9), Ni(CO)(3)(IPr) (10), Ni(CO)(3)(SIPr) (11), and Ni(CO)(3)(ICy) (12), respectively. The electronic properties of these complexes have been compared to their phosphine analogues of general formula Ni(CO)(3)(PR3) by recording their v(CO) stretching frequencies. While all of these NHCs are better donors than tertiary phosphines, the differences in donor properties between ligands 1-5 are surprisingly small. Novel, unsaturated Ni(CO)(2)(IAd) (13) and Ni(CO)(2)((IBu)-Bu-t) (14) compounds are obtained from the reaction of Ni(CO)(4) with IAd (6) and (IBu)-Bu-t (7). Complexes 13 and 14 are highly active toward substitution of the NHC as well as the carbonyl ligands. This has allowed the determination of Ni-C(NHC) bond dissociation energies and the synthesis of various unsaturated Ni(0) and Ni(II) complexes. Computational studies on compounds 8-14 are in line with the experimental findings and show that IAd (6) and (IBu)-Bu-t (7) are more bulky than IMes (1), SIMes (2), IPr (3), SIPr (4), and lCy (5). Furthermore, a method based on %V-bur values has been developed for the direct comparison of steric requirements of NHCs and tertiary phosphines. Complexes 8-14, as well as NiCl(C3H5)((IBu)-Bu-t) (16) and NiBr(C3H5)((IBu)-Bu-t) (17), have been characterized by X-ray crystallography.