Synthetic, structural, and thermochemical studies of N-heterocyclic carbene (NHC) and tertiary phosphine Ligands in the [(L)(2)Ni(CO)(2)] (L = PR3, NHC) system

Two new dicarbonyl N-heterocyclic carbene nickel(0) complexes of the type (NHC)(2)Ni(CO)(2) (NHC = ICy, [N,N'-bis(cyclohexylimidazol)-2-ylidene (2), IMes [N,N'-bis(2,4,6-trimethylphenyl)imidazol)-2-ylidene] (3)) have been prepared by a substitution reaction of (NHC)Ni(CO)(2) (NHC = (IBu)-Bu-t [N,N'-bis(tert-butylimidazol)-2-ylidene], IAd [N,N'-bis(1-adamantylimidazol)-2-ylidene]) and 2 equivalents of ICy or IMes. Single-crystal X-ray analyses confirmed the monomeric 18-electron compositions Of [(ICY)2Ni(CO)21 (2) and [(IMeS)(2)Ni(CO)(2)] (3). The greater electron-donating properties of the NHC ligands compared to tertiary phosphines are also demonstrated through calorimetric studies and enabled the determination of average bond dissociation enthalpies for Ni-L (L = NHC and tertiary phosphine).