| Abstract
| - The electron density distributions (EDD) of the redox active mixed valence trinuclear oxo-centerediron carboxylate, [Fe3O(CH2ClCOO)6(H2O)3]·3H2O, 1, and the oxidized form of 1, [Fe3O(CH2ClCOO)6(H2O)2(CH2ClCOO)]·1H2O, 2, as well as of [Fe3O(C(CH3)3COO)6 (NC5H5)3], 3, have been determined fromaccurate single-crystal X-ray diffraction data measured at 100 K (1, 2) and from extensive synchrotronradiation X-ray diffraction data measured at 28 K (3). Analysis of the EDDs shows that the central oxygenatom has a very different EDD in the mixed valence complexes (1 and 3) compared with the oxidizedcomplex (2). Furthermore, in 1 and 3 the chemical bonds between formally identical trivalent Fe atomsand the central oxygen are fundamentally different. This is in direct contrast to the FeIII-(μ3-O) bonds in theoxidized complex, which are practically identical. Analysis of the d-orbital populations on the metal sites inthe three complexes shows that the extra electron density on the FeII site primarily is distributed in a d(yz)orbital (z-axis toward the central oxygen, y-axis perpendicular to the Fe3O-plane). Presence of extra chargein the d(yz) orbital correlates with a decrease in the d(xy) population, i.e., with a depletion of charge in theequatorial region of coordination to carboxylate oxygen. The d(xy) charge depletion appears to be ofimportance for determining the active versus trapped FeIII site, and the equatorial ligands therefore havea considerable influence on the ET process. Bader topological analysis of the EDDs corroborates theconclusions drawn from the orbital population analysis, but it also provides additional knowledge about thechemical bonding in the structures. For comparison with the X-ray results, theoretical calculations werecarried out for 3 in the experimental geometry. The present information about ET processes in trinuclearoxo-centered iron complexes cannot be deduced from analysis of the molecular structures (i.e., bond lengthsand angles), and thus it is demonstrated that X-ray charge density analysis is able to reveal subtle newfeatures of significant physical and chemical importance on complex molecular systems.
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