Ab initio molecular orbital study of Fe2Cl6 and FeAlCl6

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