[Hydroxy(aryl)methylene]diphos­phonic acids,a class of drugs in bone pathology treatments,crystallize as head‐to‐head dimers

Two hydroxy­(aryl)­methyl­ene]­di­phospho­nic acids have been crystallized as dimers. The first compound, hydroxy­(phenyl)­methyl­ene]­di­phospho­nic acid monohydrate, C₇H₁₀O₇P₂·H₂O, crystallizes in the non‐centrosymmetric space group P2₁, with the two enantiomers related by a non‐crystallographic centre of inversion, while the second compound, hydroxy(4‐nitro­phenyl)­methyl­ene]­di­phospho­nic acid tetra­hydro­furan disolvate, C₇H₉NO₉P₂·2C₄H₈O, crystallizes in the centrosymmetric space group P2₁/c and uses the centre of symmetry to form the same dimer.