Ab initio molecular orbital study of Fe2Cl6 and FeAlCl6

Ab initio molecular orbital theory was used to determine the equilibrium structure and vibrational frequencies of Fe2Cl6 and FeAlCl6. The equilibrium structure the Fe2Cl6 dimer has D-2h symmetry with a planar arrangement of the four membered {FeClbrFeClbr} ring, similar to the Al2Cl6 dimer. The calculated bond distances and vibrational frequencies are in good agreement with experiment. The potential energy surface for the puckering of the {FeClbrFeClbr} ring is extremely flat. This prevents an unambiguous assignment of either D-2h or C-2v symmetry to the Fe2Cl6 structure in electron diffraction measurements. The FeAlCl6 molecule is found to have a C-2v structure similar to Fe2Cl6 with vibrational frequencies in good agreement with experiment. (C) 2000 Elsevier Science B.V. All rights reserved.