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 vCO = 1950±10, vFeC=530±10, and v bend=330±50 cm-1 in the 3Σ - state, and vCO=1990±15, vFeC= 460±15, and vbend=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.