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

The ground state, GS, geometries for Fe1,2(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.