Unusual variations in the incidence of Z? > 1 in oxo-anion structures

A series of Cambridge Structural Database studies show that ionic species generally form low Z' structures, even in those cases where charge assisted hydrogen bonding is a key feature, e.g. oxo-anion complexes. By introducing a competing pi-pi stacking interaction, two oxo-anion compounds are shown to crystallise with more than one molecule in the asymmetric unit, including the first hydrogen phosphate containing structure to have Z' > 2.