SK-edge X-ray absorption studies of tetranuclear iron-sulfur clusters: mu-sulfide bonding and its contribution to electron delocalization

X-ray absorption spectroscopy (XAS) at the sulfur (similar to 2470 eV) and chlorine (similar to 2822 eV) K-edges has been applied to a series of 4Fe-4 S model complexes. These are compared to 2Fe-2S model complexes to obtain i nsight into the localized ground state in the mixed-valence dimer versus th e delocalized ground state in the mixed-valence tetramer. The preedges of h ypothetical delocalized mixed-valence dimers [Fe2S2](+) are estimated using trends from experimental data and density functional calculations, for com parison to the delocalized mixed-valence tetramer [Fe4S4](2+). The differen ces between these two mixed-valence sites are due to the change of the sulf ide-bridging mode from mu (2) to mu (3). The terminal chloride and thiolate ligands are used as spectator ligands for the electron density of the iron center. From the intensity of the preedge, the covalency of the terminal l igands is found to increase in the tetramer as compared to the dimer. This is associated with a higher effective nuclear charge on the iron in the tet ramer (derived from the energies of the preedge). The mu (3)-bridging sulfi de in the tetramer has a reduced covalency per bond (39%) as compared to th e mu (2)-bridging sulfide in the dimer (51%). A simple perturbation model i s used to derive a quadratic dependence of the superexchange coupling const ant J on the covalency of the metal ions with the bridging ligands. This re lationship is used to estimate the superexchange contribution in the tetram er (J = -156 cm(-1)) as compared to the mixed-valence dimer (J = -360 cm-l) . These results, combined with estimates for the double exchange and the vi bronic coupling contributions of the dimer sub-site of the tetramer, lead t o a delocalized S-t = 9/2 spin ground state for the mixed-valence dimer in the tetramer. Thus, the decrease in the covalency, hence the superexchange pathway associated with changing the bridging mode of the sulfides from mu (2) to mu (3) On going from the dimer to the tetramer, significantly contri butes to the delocalization of the excess electron over the dimer sub-site in the tetramer.