A study of FeCO^- and the ³Σ^- and ⁵Σ^- states of FeCO by negative ion photoelectron spectroscopy

The 488 and 514 nm negative ion photoelectron spectra of FeCO^-, obtained at an instrumental resolution of 5 meV (40 cm^-1), show vibrationally resolved transitions from the anion ground state to the ground state and a low-lying excited state of the neutral molecule. The ground state of FeCO is assigned as the ³Σ^- state and the excited state, lying 1135±25 cm^-1 higher in energy, as the ⁵Σ^- state. The fundamental vibrational frequencies are v_CO = 1950±10, v_FeC=530±10, and v _bend=330±50 cm^-1 in the ³Σ ^- state, and v_CO=1990±15, v_FeC= 460±15, and v_bend=180±60 cm^-1 in the ⁵Σ^- state. Principal force constants are estimated from these results. Based on a Franck-Condon analysis of the spectrum and other considerations, the Fe-C bond is determined to be 0.15 ± 0.04 Å shorter, and the C-O bond 0.05±0.02 Å longer, in the ³Σ^- state than in the ⁵Σ ^- state. These results demonstrate the importance of sdσ hybridization in reducing the σ repulsion between the metal 4s electron and the CO 5σ lone pair, a mechanism that is available only when the electrons in the singly occupied 3dσ and 4s orbitals are singlet coupled as in the ³Σ^- state. The FeCO^- anion displays a high Fe-C stretching frequency (465±10 cm^-1), as well as an asymptotic Fe-CO bond energy, a bending frequency (230±40 cm^-1) and equilibrium bond lengths intermediate between those in the ³Σ^- and ⁵Σ^- states. Since the FeCO^- ground state is assigned as a ⁴Σ ^- state in which the extra electron occupies a σ orbital, these results indicate that the increased σ repulsion is partially offset by stronger metal-CO π bonding in the anion. The electron affinity of FeCO is measured to be 1.157±0.005 eV.