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(PMe<sub>3</sub>)<sub>4</sub>H<sub>2</sub>X<sub>2</sub> (X = F, Cl, Br, I) and W(PMe<sub>3</sub>)<sub>4</sub>H<sub>2</sub>F(FHF)

The molecular structures of the eight-coordinate tungsten hydride complexes W(PMe<sub>3</sub>)<sub>4</sub>H<sub>2</sub>X<sub>2</sub> (X = F, Cl, Br, I) and W(PMe<sub>3</sub>)<sub>4</sub>H<sub>2</sub>F(FHF) have been determined by single-crystal X-ray diffraction; W(PMe<sub>3</sub>)<sub>4</sub>H<sub>2</sub>Cl<sub>2</sub> and W(PMe<sub>3</sub>)<sub>4</sub>H<sub>2</sub>F(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 PMe<sub>3</sub> ligands in which two of the PMe<sub>3</sub> ligands are displaced over the halide substituents. However, the initial structures derived for both W(PMe<sub>3</sub>)<sub>4</sub>H<sub>2</sub>Cl<sub>2</sub> and W(PMe<sub>3</sub>)<sub>4</sub>H<sub>2</sub>F(FHF) did <i>not</i> exhibit the aforementioned geometry, but were based on an arrangement in which the two <i>transoid</i>-PMe<sub>3</sub> ligands are displaced toward the two <i>cis</i>-PMe<sub>3</sub> groups, rather than tilted toward the chloride ligands. Interestingly, the unexpected structures for W(PMe<sub>3</sub>)<sub>4</sub>H<sub>2</sub>Cl<sub>2</sub> and W(PMe<sub>3</sub>)<sub>4</sub>H<sub>2</sub>F(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 <i>transoid</i>-PMe<sub>3</sub> 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 <i>composite</i> 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 <i>R</i> 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 <i>true</i> polar configuration.