Molecular structures for FeS4(-/0) as determined from an ab initio study of the anion photoelectron spectra.

For the purpose of assigning the photoelectron spectra of the FeS4(-) molecular entity, geometric and electronic structures of low-lying FeS4(-/0) isomers were investigated at the B3LYP, CASPT2, and RCCSD(T) computational quantum chemical levels. The anionic ground state is predicted to be the (4)B1g state of the D2h (η(2)-(S2))2Fe(-) isomer with two S2(2-) molecular ligands side-on bond in a D2h fashion to iron, which has an oxidation state of +3. The experimental photoelectron spectra of FeS4(-) were successfully assigned as originating from this isomer. The composed lowest energy X band is the result of ionizations to the (3)B3g, (5)B1u, and (5)B1g states. Analyses of the CASSCF orbitals indicated an almost degeneracy of the nonbonding 3d orbitals of iron and the π* orbitals of S2(2-). All the experimental observed higher ionization energy bands could also be theoretically assigned as originating from the proposed anion ground state by detachment of an electron from either of these iron or ligand orbitals.