Secondary bonding as a potential design tool for crystal engineering

The concept of secondary bonding in p-block structural chemistry as outlined by Alcock is introduced in the context of its application to crystal engineering. The relationship between secondary and hydrogen bonding is described and the hypothesis advanced that the analogy between these two classes of bond is a deep and close one and that therefore the tools of analysis in the study of hydrogen bonds in crystals may be applied usefully to secondary bonding. In archetypal systems based on Bi(III) and Sb(III) complexes between 70 and 90% of the possible secondary bonds are formed, indicating the robustness of this potential supramolecular synthon. The networks formed by the secondary bonds are also analogous to those formed in hydrogen bonded crystals and are likewise prone to disruption on dissolution. It seems that secondary bonds may be a valuable alternative to conventional directional interactions in the synthesis of novel molecular crystalline solids.