Size and shape characteristics of inorganic molecules and ions and their relevance to molecular packing problems

Abstract

The manner in which molecular inorganic molecules and ions pack in the solid state is important not only for the formation of crystals suitable for diffraction studies, but also contributes significantly to the conduction, magnetic and non-linear optical properties of these molecular inorganic compounds in the solid state. In order to provide a basis for discussing the role of molecular size and shape on the packing of molecular salts, the volumes, surface areas and shapes of a wide range of ions observed in co-ordination and organometallic chemistry have been calculated from the van der Waals radii of the constituent atoms. The calculated cation volumes range from 24–795 ų and for the anions from 34–467 ų and their shapes have been classified as spherical, discoidal and cylindrical on the basis of the calculated moments of inertia. The calculated packing coefficients for molecular salts containing spherical cations and anions lie between 65 and 71.5%. This relatively narrow range provides a methodology for estimating the volumes of spherical ions not given in this compilation. The surface areas of the ions were found to be linearly correlated with the volumes. The calculated volumes and surface areas may be used to calculate the effective radii of the ions. These radii are relevant to the development of radius ratio rules for inorganic molecular salts.