Packing Symmetry Altered by Weak Packing Force. Michael Y. Chiang, Dong-Liang Wang, Lien-I Wang, Wen-Shu Hwang, Department of Chemistry, National Sun Yat-Sen University, Kaohsiung, Taiwan 804, ROC.

The relationship between packing symmetry and weak intermolecular interactions is examined via packing study of two different crystalline forms of a diiron carbonyl complex with chelating thienyl ligand (C18H11Fe2NO6S). Structural analysis reveals that a weak C-H...Ph interaction (C...Ph ring centriod=3.78A) is responsible for the dimer- like arrangement within the triclinic form obtained from hexane. The monoclinic form obtained from chloroform is lacking of the above- mentioned weak interaction and it displays an alternating arrangement that maximizes the overall attraction forces in the crystal. It is a known fact that polymorphism may occur when different solvents are used for crystallization. The solvent-solute interaction may be governing the different paths of crystal growth. The current example gives us an opportunity to closely examine the major difference between packing with 21 screw axis and packing with inversion center. Inversion center symmetry is responsible for maximizing interactions within molecular pairs while screw axis fulfill the principle of overall close packing of the molecules in the unit cell. This is because that symmetry with translational motion generates alternating arrangement of molecules which allows molecules to fall into the gap between two neighboring molecules. Other symmetry considerations together with some weak packing forces is also discussed.