THEORETICAL INVESTIGATION OF THE ELECTRONIC-STRUCTURE OF THE MIXED-SANDWICH COMPLEX (ETA-5-CYCLOPENTADIENYL)(ETA-6-BENZENE)IRON AND ITS CATION

SCF MS-Xalpha calculations have been carried out for the d6 [(Cp)Fe(Bz)]+ (1) and d7 (Cp)-Fe(Bz) (2) compounds (Cp = eta5-cyclopentadienyl, Bz - eta6-benzene), and the results are compared with those previously reported for metallocenes Fe(CP)2 (3), CO(CP)2 (4), [Ni(CP)2]+ (5), and d8 Ni(CP)2 (6) using the same technique. Analysis of the electronic structure of 1 and 2 shows that whereas Cp and Bz ligands play roughly the same role in pi-ligand to metal donation, they behave differently as far as 3ddelta back-donation is concerned, the Bz system having a much more important role in this case. This is confirmed by spin-polarized (SP) calculations performed for 2 and 4, which show that the 3ddelta orbitals exhibit a much larger interaction with the Bz ligand than with the Cp group. In addition, a considerable charge relaxation occurs at the metal upon ionization of 2, as the gross charges on the iron are roughly the same in 1 and 2. When using the MS-Xalpha results to calculate spectroscopic properties, a good agreement is found between theoretical and experimental results determined for both the electron field gradient (EFG) at the metal nucleus and Mossbauer isomer shift (IS) for compounds 1-3. Similarly, both MS-Xalpha ionization and electronic excitation energies calculated for 1 and 2 compare fairly well with photoelectron and UV-visible absorption data.