The calcium‐binding properties of pamidronate,a bone‐resorption inhibitor

The title compound, calcium bis(3‐ammonio‐1‐hydroxy­propyl­idene‐1,1‐bis­phospho­nate) dihydrate, Ca²⁺·2C₃H₁₀N­O₇P₂^?·2H₂O, consists of calcium octahedra arranged in columns along the c axis and coordinated by hydrogen‐bonded molecular anions. The Ca²⁺ cation lies on a twofold axis. Pamidronate adopts a twisted conformation of the hydroxy­alkyl­amine backbone that enables the formation of an intramolecular N—H?O hydrogen bond. The molecular anion is chelating monodentate as well as bidentate, with an O?O bite distance of 3.0647 (15) Å.