Theoretical study of the structural and electronic properties of the Fen(C6H6)m, n ≤ 2; m ≤ 2 complexes

Abstract

The ground state, GS, geometries for Fe_1,2(benzene)_1,2 clusters were determined by means of all-electron calculations done with the density functional BPW91/6311++G(2d,2p) method. The stability of Fe(C₆H₆)_1,2 is accomplished by the formation of Fe–C η⁶ coordinations in the half-sandwich and sandwich GS structures, which are of lower spin, 2S = 2 (S is the total spin) than the Fe atom, 2S = 4. Departures from η⁶ bonding occur on [Fe(C₆H₆)₂]⁻, since the GS of this anion, of less symmetric sandwich geometry, presents η⁶ and η² coordination, which is mainly due to the enhanced repulsion of the adsorbed benzene units. On Fe₂(C₆H₆)_1,2 the stronger Fe₂ bond, compared to the Fe–C ones, produce rice-ball geometries, where the Fe₂ molecule, although with a longer bond length, is preserved. For example, in Fe₂(C₆H₆), Fe₂ lies perpendicular or parallel to the benzene ring depending on the charge of the complex, and in [Fe₂(C₆H₆)₂]^±0, ±1 the benzene ligands are placed above and beneath the molecular axis of Fe₂, producing highly compact structures. Multiple decker sandwich states, where Fe₂ is not retained, are located more than 20 kcal mol⁻¹ above the GS levels. Electron affinities, agreeing well with experimental results, ionization and binding energies, and vibrational frequencies were also determined, providing insight on the complexes.