FALSE MINIMA IN X-RAY STRUCTURE SOLUTIONS ASSOCIATED WITH A PARTIAL POLAR AMBIGUITY - SINGLE-CRYSTAL X-RAY AND NEUTRON-DIFFRACTION STUDIES ON THE 8-COORDINATE TUNGSTEN HYDRIDE COMPLEXES, W(PME3)(4)H2X2 (X=F, CL, BR, I) AND W(PME3)(4)H2F(FHF)

The molecular structures of the eight-coordinate tungsten hydride comp lexes W(PMe3)(4)H2X2 (X = F, Cl, Br, I) and W(PMe3)(4)H2F(FHF) have be en determined by single-crystal X-ray diffraction; W(PMe3)(4)H2Cl2 and W(PMe3)(4)H2F(FHF) have also been analyzed by single-crystal neutron diffraction, thereby accurately locating the positions of the hydride ligands. The structures of all of these complexes are similar and are based on a trigonal dodecahedron, with a distorted tetrahedral array o f PMe3 ligands in which two of the PMe3 ligands are displaced over the halide substituents. However, the initial structures derived for both W(PMe3)(4)H2Cl2 and W(PMe3)(4)H2F(FHF) did nos exhibit the aforementi oned geometry, but were based on an arrangement in which the two trans oid-PMe3 ligands are displaced toward the two cis-PMe3 groups, rather than tilted toward the chloride ligands. Interestingly, the unexpected structures for W(PMe3)(4)H2Cl2 and W(PMe3)(4)H2F(FHF) were discovered to be the result of an artifact due to the presence of a heavy atom i n a polar space group, which allowed the X-ray structure solutions to refine into most deceptive false minima. Specifically, for the structu res corresponding to the false minima, the transoid-PMe3 ligands were incorrectly located in positions that are related to their true locati ons by reflection perpendicular to the polar axis. In effect, the inco rrect molecular structures are a composite of the two possible true po lar configurations which are related by a reflection perpendicular to the polar axis, i.e. a ''partial polar ambiguity''. Of most importance , the solutions corresponding to the false minima are characterized by low R values and well-behaved displacement parameters, so that it is not apparent that the derived structures are incorrect. Thus, for spac e groups with a polar axis, it is necessary to establish that all of t he atoms in the asymmetric unit belong to a single true polar configur ation.