Quantum chemical study of the geometric and electronic structure of the FeC2 molecule

The FeC2 molecule in its different geometric conformations and electronic states has been studied using the DFT (B3LYP) and CASSCF/CASPT2 methods. Both approaches predict the cyclic (C2v) structure to be more stable than the linear (C∞v) conformation. The ground state has been found to be a quintet, which can be formally regarded as an ionic Fe2+–C22− complex, with two 4s electrons of Fe transferred to the C2 ligand and the remaining six valence electrons distributed among the split d orbitals. For the low-lying excited states of the C2v conformation, there is a good semi-quantitative agreement between the B3LYP and the CASPT2 results.