Goal: Many crystal structures have been refined in the past 100 years, and a large amount of information concerning interatomic distances in the solid state is available. In the 1960s and 1970s, a considerable amount of work was done trying to understand the reasons underlying variations in mean bond-length in crystals. This resulted from the improving precision of structure refinements which began showing variations in mean bond-length that significantly exceeded experimental error. Several factors were examined as possible sources of this variation, and many studies were reported as ‘reasonably successful’ in correlating variation in mean bond length with one or more possible causal factors, e.g. variation in mean coordination number of the bonded anions, variation in mean electronegativity of the next-nearest-neighbour cations, dispersion of bond lengths about their mean value (distortion). However, these studies were typically limited to a single configuration of the oxidation state and coordination number of an ion, and often consisted of few data.

Since then, the proliferation and increasing efficiency of single-crystal diffractometers have produced a large amount of crystal-structure data, and the advent of data bases (particularly the ICSD: Inorganic Crystal Structure Database) has provided a major opportunity to examine factors affecting the variation in individual and mean bond-lengths with a large amount of data. An advantage of working with a large number of ion pairs and a large amount of data is that it allows examination of subtle differences between the shapes of various distributions (e.g. bond-length distributions, mean-bond-length distributions) for various configurations of ions, which reflect differences in their bonding behaviour. These differences typically arise from either structural and/or electronic effects, and are well known for extreme examples such as [6]-coordinated Cu2+ and [6]-, [7]- and [8]-coordinated U6+; however, more subtle deviations from unimodality are to be expected for the bond-length distributions of other ion configurations that are involved in related electronic or structural effects. The motivation for this work is twofold: (1) The factors that affect bond distances are of continuing interest to all who work on crystal structures and their properties, and a comprehensive analysis of all the data should lead to increased understanding of those factors. In addition, a comprehensive knowledge of the observed variation in bond lengths is critically important in assessing the validity of computational results on possible atomic arrangements and identifying unusual stereochemical features in newly solved or refined crystal structures.