virtual issues

The past two decades have witnessed a veritable explosion of reports detailing the solid-state structures of coordination compounds consisting of conceptually infinite one-dimensional chains, two-dimensional nets and three-dimensional frame­works. The concepts involved are relatively simple; crystalline coordination polymers are most often formed from solutions containing metal ions and bridging ligands. The seemingly unlimited opportunities for preparing novel coordination polymers arise not only from obvious choices that influence structural topology (i.e. bridging ligand geometry and metal ion coordination mode) but also from a range of other factors, some of which can be quite subtle. The collection of articles selected for this virtual issue serves to showcase the amazing diversity of the field.

For the validation of absolute-structure determination, over the years there has been a heavy reliance on the statistics (values and standard uncertainties) of derived parameters or something equivalent, and little study of the fit of the model to the observed quantities. There has been little questioning of the quality of crystals, data collection and correction methodologies. Clearly, absolute-structure determination is still a rich field, wide open for development, especially for light-atom compounds of great importance to the pharmaceutical and chemical industries.

Polymorphism, particularly for molecular materials, was perhaps once considered a curiosity. Nowadays, polymorphism has become a crucial issue for the pharmaceutical industry. Different polymorphs of a drug can have quite different properties in terms of solubility, bioavailability, shelf life, ease of production, and other physical properties. Developments in crystal structure prediction in recent years have also highlighted hitherto unknown polymorphs.