Bonding and Magnetism of Fe<sub>6</sub>−(C<sub>6</sub>H<sub>6</sub>)<sub><i>m</i></sub>, <i>m</i> = 1, 2

The interactions of one and two benzene molecules with the superparamagnetic Fe<sub>6</sub> cluster were studied by means of gradient-corrected density functional theory. The ground state, GS, of bare Fe<sub>6</sub> presents a distorted octahedral structure with 2<i>S</i> = 20; <i>S</i> is the total spin. For the calculated 2<i>S</i> = 16 GS of the neutral Fe<sub>6</sub>−C<sub>6</sub>H<sub>6</sub> complex, as well as in the positive and negative ions both with 2<i>S</i> = 15, the benzene unit is adsorbed on one axial Fe<sub>a</sub> atom. The 2<i>S</i> = 14 GS for Fe<sub>6</sub>−(C<sub>6</sub>H<sub>6</sub>)<sub>2</sub> resembles a sandwich structure, with the metal Fe<sub>6</sub> cluster separating the benzene rings that are bonded symmetrically on the two axial sites of Fe<sub>6</sub>. The binding is accounted for by electrostatic interactions and by 3d−π bonds, as revealed by the molecular orbitals. Though each C−Fe bond is weak, η<sup>6</sup> coordinations were indicated by the topology of the electronic density. The 3d−π bonding is reflected by the adiabatic ionization energies and electron affinities, which are smaller than those of bare Fe<sub>6</sub>. The computed IR spectra show vibrational bands near those of bare benzene; some forbidden IR modes in benzene and in Fe<sub>6</sub> become IR active in Fe<sub>6</sub>−(C<sub>6</sub>H<sub>6</sub>)<sub>1,2</sub>. The results show a strong perturbation of the electronic structure of Fe<sub>6</sub>. The decrease of its magnetic moment implies that the magnetic effects play an important role in the adsorption of benzene.