False Minima in X-ray Structure Solutions Associated with a “Partial Polar Ambiguity”: Single Crystal X-ray and Neutron Diffraction Studies on the Eight-Coordinate Tungsten Hydride Complexes, W(PMe3)4H2X2 (X = F, Cl, Br, I) and W(PMe3)4H2F(FHF)
journal contributionposted on 28.04.1998 by Vincent J. Murphy, Daniel Rabinovich, Tony Hascall, Wim T. Klooster, Thomas F. Koetzle, Gerard Parkin
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The molecular structures of the eight-coordinate tungsten hydride complexes W(PMe3)4H2X2 (X = F, Cl, Br, I) and W(PMe3)4H2F(FHF) have been determined by single-crystal X-ray diffraction; W(PMe3)4H2Cl2 and W(PMe3)4H2F(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 of PMe3 ligands in which two of the PMe3 ligands are displaced over the halide substituents. However, the initial structures derived for both W(PMe3)4H2Cl2 and W(PMe3)4H2F(FHF) did not exhibit the aforementioned geometry, but were based on an arrangement in which the two transoid-PMe3 ligands are displaced toward the two cis-PMe3 groups, rather than tilted toward the chloride ligands. Interestingly, the unexpected structures for W(PMe3)4H2Cl2 and W(PMe3)4H2F(FHF) were discovered to be the result of an artifact due to the presence of a heavy atom in a polar space group, which allowed the X-ray structure solutions to refine into most deceptive false minima. Specifically, for the structures corresponding to the false minima, the transoid-PMe3 ligands were incorrectly located in positions that are related to their true locations by reflection perpendicular to the polar axis. In effect, the incorrect molecular structures are a composite of the two possible true polar 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 space groups with a polar axis, it is necessary to establish that all of the atoms in the asymmetric unit belong to a single true polar configuration.