[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, C(7)H(10)O(7)P(2).H(2)O, crystallizes in the non-centrosymmetric space group P2(1), with the two enantiomers related by a non-crystallographic centre of inversion, while the second compound, [hydroxy(4-nitrophenyl)methylene]diphosphonic acid tetrahydrofuran disolvate, C(7)H(9)NO(9)P(2).2C(4)H(8)O, crystallizes in the centrosymmetric space group P2(1)/c and uses the centre of symmetry to form the same dimer.