Formally Ferric Heme Carbon Monoxide Adduct.

Formally ferric carbonyl adducts are reported in a series of thiolate-bound iron porphyrins. Resonance Raman data indicate the presence of both Fe-S and Fe-CO bonds, and EPR data of this S = 1/2 species indicate a ligand-based electron hole, giving this complex an Fe(II)-thiyl radical electronic ground state. The FTIR data show that the C-O vibrations are substantially higher than in the corresponding ferrous-thiolate CO adducts. DFT calculations reproduce the spectroscopic features and indicate that backbonding to the low lying π* orbitals of the bound CO stabilizes the Fe 3d orbitals resulting in a stabilization of the ferrous-thiyl radical ground state compared to the five-coordinate ferric-thiolate precursor complexes. Access to stable thiyl radicals will help understand these elusive species that are mostly encountered as short-lived reactive reaction intermediates.