Sulfur vs. Selenium as Bridging Ligand in Di-Iron Complexes: A Theoretical Analysis

The replacement of S with Se is a useful technique for studying iron-sulfur clusters. The substitution is typically considered a small perturbation to the electronic structure of the cluster. The advantage is that element specific techniques, such as X-ray absorption and emission spectroscopy, can be used to selectively investigate the environment of the Se atoms in the cluster. In this work, the effect of this perturbation has been studied quantitatively with the help of high-level electronic structure calculations. We present a systematic comparison of iron-sulfur monomers and dimers and their Se analogs using wave function-based ab initio methods. First, the local electronic structure of the Fe-S and Fe-Se bonds is studied using ab initio ligand field theory (AILFT) in conjunction with the angular overlap model (AOM). Second, the effect of Se substitution on the low-energy spectrum in homo-valent (Fe3+Fe3+) and the mixed-valent (Fe2+Fe3+) iron-sulfur dimers is investigated in detail. We find that Se-based ligands generally induce a weaker ligand field, possess a smaller donor strength, and reduce the coupling between the iron centers compared to their S counterparts. Furthermore, the differences between S and Se can affect the energy ordering of electronic states in cases with close-lying electronic states.