Metal-based anion receptors: an application of second-sphere coordinationPart of a themed issue on the supramolecular chemistry of anionic species

This tutorial review describes metal-based anion receptors in the context of first- and second-sphere coordination. Selected examples (case studies) are used to show how metal ions can be used to (1) organise hydrogen-bond donor sites on ligands in order to bind anions, (2) influence the conformation of ligands to enhance anion binding, (3) alter the binding conformation of a receptor depending on the solvent and (4) develop a receptor to target a specific anion (sulfate) for sequestration. Second in name only! A robust transition metal complex can utilise significant second-sphere coordination from H-donor groups on independent ligands (L) to form a binding site for anion (A) recognition.