Is it possible to synthesize a low-valent transition metal complex with a neutral carbon atom as terminal ligand?: A theoretical study of (CO)4FeC

Quantum chemical calculations at the NL-DFT (B3LYP) and CCSD(T) levels of theory have been carried out for the carbon complex (CO)(4)FeC. The bonding situation was analyzed with the NBO partitioning scheme and with the topological analysis of the electron density distribution. The results have been compared with the bonding situations in (CO)(4)FeCH2, I(CO)(3)FeCH, and Fe(CO)(5). The trigonal-bipyramidal complex (CO)4FeC with an axial Fe-C bond is a minimum on the singlet potential energy surface, while the equatorial form is a transition state. The Fe-C bond has a large dissociation energy, D-e = 84.1 kcal/mol at B3LYP/II and D-e = 94.5 kcal/mol at CCSD(T)/II. The carbon ligand is a strong sigma-acceptor and an even stronger sigma-donor. The analysis of the electronic structure suggests that (CO)(4)FeC should behave like a carbon nucleophile. Geometry optimization of the donor-acceptor complex (CO)(4)FeC-BCl3 yielded a strongly bonded compound, which has a calculated C-B bond energy of D-e = 25.6 kcal/mol at B3LYP/II. Lewis acid stabilized carbon complexes such as (CO)(4)FeC-BCl3 might become isolated under appropriate conditions.