pi stacking and the co-ordinate bond: Sometimes conflicting factors in molecular recognition, as revealed in the structures of metal picrates

Crystallographic studies of anhydrous and hydrated 2,4,6-trinitrophenoxides, 'picrates', of metal ions from the main-group, transition, lanthanide and actinide metal series show that picrate may be a ligand of comparable donor ability to hydroxide and often is one of remarkably high denticity, but that these characteristics do not guarantee its co-ordination in preference to water. It is argued that this may be attributable to its 'self-coordinating' ability through pi stacking, a structural feature which is universally observed regardless of whether picrate is also co-ordinated, and that the pi stacking interaction energy must be comparable to the difference between hydroxide and water co-ordinate bond energies.