N,N'-Dimethyl­piperazinium(2+) bis­[methyl­enehydrogendiphospho­nate(1-)]

The asymmetric unit of the title compound, C₆H₁₆N₂²⁺·2(HO)₂(O)PCH₂P(O)₂(OH)^-, contains two singly charged diphospho­nate anions and two half-cations, each doubly charged cation lying on an inversion centre. Single deprotonation of methyl­enediphospho­nic acid to give a salt with an organic amine is unprecedented, double deprotonation being normal. All N-H and O-H groups act as hydrogen-bond donors in the crystal structure, with unproton­ated O atoms as the acceptors, giving a three-dimensional network.