The size and shape of molecular ions and their relevance to the packing of the hexafluorophosphate salts

Abstract

The volumes, surface areas, effective radii and shapes of inorganic ions have been used to rationalise the solid-state structures of a number of [PF₆]^– salts. The effective radii of the cluster ions, derived from the volume contained within the van der Waals sphere of the atoms, may be used to calculate radius ratios for the salts which are used to interpret the observed solid-state structures. The distortions from the idealised inorganic salt structures have been related to the non-spherical nature of the ions, which have been calculated from their moments of inertia. A method for graphically displaying the shape of the ions using a thermal ellipsoid plotting program has been developed. Finally the lattice energies of the salts have been calculated using the atom–atom potential method and the results show that the charge interactions between polarised ions can produce a structure that deviates greatly from the close-packed ideal.