Electron density distributions of redox active mixed valence carboxylate bridged trinuclear iron complexes

The electron density distributions (EDD) of the redox active mixed valence trinuclear oxo-centered iron carboxylate, [Fe3O(CH2ClCOO)(6)(H2O)(3)].3H(2)O, 1, and the oxidized form of 1, [Fe3O(CH2ClCOO)(6).(H2O)(2)(CH2ClCOO)].1H(2)O, 2, as well as of [Fe3O(C(CH3)(3)COO)(6) (NC5H5)(3)], 3, have been determined from accurate single-crystal X-ray diffraction data measured at 100 K (1, 2) and from extensive synchrotron radiation X-ray diffraction data measured at 28 K (3). Analysis of the EDDs shows that the central oxygen atom has a very different EDD in the mixed valence complexes (11 and 3) compared with the oxidized complex (2). Furthermore, in 1 and 3 the chemical bonds between formally identical trivalent Fe atoms and the central oxygen are fundamentally different. This is in direct contrast to the Fe-III-(mu(3)-O) bonds in the oxidized complex, which are practically identical. Analysis of the cl-orbital populations on the metal sites in the three complexes shows that the extra electron density on the Fe-II 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 charge in the d(yz) orbital correlates with a decrease in the d(xy) population, i.e., with a depletion of charge in the equatorial region of coordination to carboxylate oxygen. The d(xy) charge depletion appears to be of importance for determining the active versus trapped Fe-III site, and the equatorial ligands therefore have a considerable influence on the ET process. Bader topological analysis of the EDDs corroborates the conclusions drawn from the orbital population analysis, but it also provides additional knowledge about the chemical bonding in the structures. For comparison with the X-ray results, theoretical calculations were carried out for 3 in the experimental geometry. The present information about ET processes in trinuclear oxo-centered iron complexes cannot be deduced from analysis of the molecular structures (i.e., bond lengths and angles), and thus it is demonstrated that X-ray charge density analysis is able to reveal subtle new features of significant physical and chemical importance on complex molecular systems.