Bonding and magnetism of Fe(6)-(C(6)H(6))(m), m = 1, 2.

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