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) 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 = 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).