Ammonium 1-hydroxy-2-(2-pyridinio)ethane-1,1-diyldiphosphonate dihydrate and potassium 1-hydroxy-2-(2-pyridinio)ethane-1,1-diyldiphosphonate dihydrate.

The crystal structures of the title compounds, ammonium risedronate dihydrate, NH4+.C7H10NO7P2-.2H2O, (I), and potassium risedronate dihydrate, K+.C7H10NO7P2-.2H2O, (II), have been determined from single-crystal X-ray data collected at 120 K. Compound (I) forms a three-dimensional hydrogen-bonded network which connects the ammonium and risedronate ions and the water molecules. In compound (II), the K+ ions are seven-coordinated in a capped distorted trigonal prism. The coordination polyhedra form chains by corner-sharing, and these chains are connected by phosphonate groups into layers in the ac plane. The layers are stacked and connected by hydrogen bonds in the b direction. The risedronate conformation is determined by intramolecular interactions fine-tuned by crystal packing effects. All H-atom donors in both structures are involved in hydrogen bonding, with D...A distances between 2.510 (2) and 3.009 (2) A.