Anions in crystal engineering.

This tutorial review presents a current account of anions in crystal engineering, organized around two main questions: (i) how do anions influence and control crystal structures, and (ii) how do crystal environments recognize and select anions? The first part pertains to deliberate assembly of new crystalline materials using anionic components, by taking advantage of the strong and directional interactions of anions in the solid state. Along this line, the various structural roles of anions in crystals are examined. The second question is related to selective separation of anions by crystallization, by exploiting chemical recognition phenomena in the well-defined and highly structured environment inside crystals.