Isocyanide versus nitrile ligands and methyl versus trifluoromethyl substituents in metal carbonyl chemistry

Abstract

The relative energies of the nitrile and isocyanide metal carbonyl complexes LM(CO)n (L=RCN and RNC; R=CH3 and CF3; M=Cr, n=5; M=Ni, n=3; M=Fe, n=4) have been compared using density functional theory. For the ligand pair CH3CN/CH3NC the acetonitrile metal carbonyl complexes lie at consistently lower energies than the isomeric methyl isocyanide complexes with energy differences of ∼10kcal/mol for the Cr and Ni complexes and ∼5kcal/mol for the Fe complexes. This relative energy order is reversed for the perfluorinated ligand pair CF3CN/CF3NC for which the trifluoromethyl isocyanide metal carbonyl complexes have lower energies than the isomeric trifluoroacetonitrile complexes. This can be related to the high π-acceptor ability of CF3NC as indicated by the ν(CO) and ν(CN) frequencies as well as by Morokuma energy decomposition analysis. However, the energy differences for the fluorinated derivatives are rather small (1–3kcal/mol) for the Cr and Ni complexes but somewhat larger (6–8kcal/mol) for the Fe complexes. In the Cs trigonal bipyramidal iron carbonyl structure eq-(CF3NC)Fe(CO)4 the normally linear C–N–C(F3) unit is bent to ∼137° thereby indicating unusually strong Fe→CNCF3 π back-bonding. For the uncomplexed nitrile/isocyanide pairs CX3CN/CX3NC (X=H, F) previous experimental and theoretical studies indicate the nitrile to lie ∼23kcal/mol in energy below the isomeric isocyanide. Thus the current research suggests that complexation with metal carbonyl fragments stabilizes isocyanides relative to the isomeric nitriles.