Theoretical study of the structural and electronic properties of the Fe(6)-(C(6)H(6))(m), m = 3, 4, complexes.

The adsorption of benzene on the magnetic Fe(6) cluster was studied by means of first principles all-electron calculations done with gradient corrected density functional theory. In the M = 2S + 1 = 13 (S is the total spin) ground state (GS) of Fe(6)-(C(6)H(6))(3) each benzene is bonded with one Fe atom, forming eta(6) coordinations with C-Fe contacts of 2.12-2.17 A; though the Fe(6) cluster structure is preserved, it presents more distortion than in bare Fe(6). The M = 13 GS of Fe(6)-(C(6)H(6))(4) shows a more distorted geometry with three eta(6) and one eta(2) coordinations, as the bonding with the fourth benzene was reduced to two C-Fe bonds. Thus, Fe(6)-(C(6)H(6))(4) may be viewed as a Fe(6) core covered by a layer of benzene molecules. The d-pi bonding interactions are clearly reflected by the estimated adiabatic ionization energies (4.60 and 4.42 eV for m = 3 and 4, respectively), because they are significantly smaller than that of bare Fe(6), 6.15 eV. The adiabatic electron affinities also are diminished clearly, 1.02 and 1.13 eV, for m = 3 and 4, respectively, as compared to that of Fe(6), 1.61 eV. The magnetic moments of the Fe(6)-(C(6)H(6))(3,4) complexes are strongly quenched, by 8.0 magneton bohrs (mu(B)), with respect to the value, 20.0 mu(B), of the isolated Fe(6) cluster. Lastly, the vibrational spectra show IR bands placed near those of free benzene and several forbidden IR modes of benzene turn IR active in the reduced symmetry of the complexes.