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)

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