Quantitative Account of the Bonding Properties of a Rubredoxin Model Complex [Fe(SCH3)4]q, q =-2,-1,+2,+3

Iron-sulfur clusters play important roles in biology as parts of electron-transfer chains and catalytic cofactors. Here, we report a detailed computational analysis of a structural model of the simplest natural iron-sulfur cluster of rubredoxin and its cationic counterparts. Specifically, we investigated adiabatic reduction energies, dissociation energies, and bonding properties of the low-lying electronic states of the complexes [Fe(SCH3)(4)](2-/1-/2+/3+) using multireference (CASSCF, MRCISD), and coupled cluster [CCSD(T)] methodologies. We show that the nature of the Fe-S chemical bond and the magnitude of the ionization potentials in the anionic and cationic [Fe(SCH3)(4)] complexes offer a physical rationale for the relative stabilization, structure, and speciation of these complexes. Anionic and cationic complexes present different types of chemical bonds: prevalently ionic in [Fe(SCH3)(4)](2-/1-) complexes and covalent in [Fe(SCH3)(4)](2+/3+) complexes. The ionic bonds result in an energy gain for the transition [Fe(SCH3)(4)](2-)->[Fe(SCH3)(4)](-) (i.e., Fe-II -> Fe-III) of 1.5 eV, while the covalent bonds result in an energy loss for the transition [Fe(SCH3)(4)](2+) -> [Fe(SCH3)(4)](3+) of 16.6 eV, almost half of the ionization potential of Fe2+. The ionic versus covalent bond character influences the Fe-S bond strength and length, that is, ionic Fe-S bonds are longer than covalent ones by about 0.2 angstrom (for Fe-II) and 0.04 angstrom (for Fe-II). Finally, the average Fe-S heterolytic bond strength is 6.7 eV (Fe-II) and 14.6 eV (Fe-III) at the RCCSD(T) level of theory.