Structure and bonding in monomeric iron(III) complexes with terminal oxo and hydroxo ligands

We report on the structure and bonding in the title iron(III) complexes, containing the tris[(N'-tert-butylureayl)-N-ethyl]amine ligand, with density functional theory techniques. In agreement with the experimental data, a high-spin electronic state is favored for all of the systems we considered. H bonds between the terminal oxo and hydroxo ligands and NH groups present in the organic ligand coordinated to the metal have a remarkable effect on the overall coordination geometry. In fact, the structure of model complexes without H bonds shows shorter Fe-O bond lengths. This is a consequence of the ability of the H bonds to stabilize a remarkable amount of electron density localized on the terminal oxo and hydroxo ligands. Energy analysis indicates that each H bond stabilizes the nonheme complexes by roughly 35 kJ/mol. Molecular orbital analysis indicates a reduction of two Fe-O bonding electrons on going from a complex with a terminal oxo ligand to a complex with a terminal hydroxo ligand. This reduction in the number of bonding electrons is also supported by frequency analysis.