Synthetic, Structural, and Thermochemical Studies of N-Heterocyclic Carbene (NHC) and Tertiary Phosphine Ligands in the [(L)2Ni(CO)2] (L = PR3, NHC) System

Two new dicarbonyl N-heterocyclic carbene nickel(0) complexes of the type (NHC)2Ni(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 = I t Bu [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) 2 Ni(CO)2] (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).