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

Abstract

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 νCO=1950±10, νFeC=530±10, and νbend=330±50 cm−1 in the 3Σ− state, and νCO=1990±15, νFeC=460±15, and νbend=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.