Carbonyl complexes of Ni(0) incorporating two N-heterocyclic carbenes of the type Ni(CO)2(NHC)2 (NHC = ICy [N, N'-bis(cyclohexylimidazol)-2-ylidene], IMes [N, N'- bis(2, 4, 6-trimethylphenyl)-imidazol)-2-ylidene]) have been prepared. The complexes Ni(CO)2(ICy)2 (8) and Ni(CO)2(IMes)2 (9) have been synthesized and characterized by single crystal X-ray diffraction. The enthalpy of substitution reactions of Ni(CO)2(NHC) (NHC = ItBu [N, N'-bis(tert-butylimidazol)-2-ylidene], IAd [N, N'-bis(1- adamentylimidazol)-2-ylidene]) with NHC and tertiary phosphine ligands leading to the formation of Ni(CO)2(L)2 (L = NHC, PR3) complexes have been determined. The solution calorimetric investigations reiterate the greater electron donating property of the NHC ligands compared to tertiary phosphines. Thermochemical studies of the substitution reactions of Ni(CO)2(NHC) (NHC = ItBu, IAd) forming complexes (8) and (9) led to the determination of average bond dissociation energy of Ni-NHC (NHC = ICy, IMes) and Ni-P (P = PCy3, PPh3, P(p-Tol)3, P(m-Tol)3).