False Minima and the Perils of a Polar Axis in X-ray Structure Solutions: Molecular Structures of W(PMe_3)_4H_2X_2 (X = F, Cl, Br) and W(PMe_3)_4H_2F_2(H_2O)

摘要

We have demonstrated that molecules which crystallize in polar space groups may suffer from structure solutions that refine into deceptive false minima. Although the existence of false minima is not without precedence, the examples described here are remarkable in the sense that the false structures are dramatically different from their true structures but are nevertheless characterized by low R values and well-behaved displacement parameters. It is likely that this effect will be most pronounced when the X-ray scattering is dominated by a single atom. Thus, in addition to the well-known requirement of ensuring that the correct sense of a polar axis has been selected, it is critical to establish that all of the atoms in the asymmetric unit of a polar space group belong to a single true polar configuration and that the correct molecular geometry is established.