Polymeric methylenebis(diphenylphosphine oxide) hydrogen triiodide

The conformation of the cationic part of the title compound, {(C₆H₅)₂POH_0.5}₂CH₂]I₃ or dppmO₂H⁺·I₃^? (dppm is di­phenyl­phosphino­methane), is determined by hydrogen bonds between cations of monoprotonated (C₆H₅)₂P(=O)]₂CH₂ (dppmO₂). Symmetric P=O?H?O=P bridging, with H atoms lying on centres of inversion, leads to chain‐like polymeric cations, (dppmO₂H⁺)_x, made up of H?OP(C₆H₅)₂—CH₂—(C₆H₅)₂PO? moieties. These are, in turn, cross‐connected by non‐classical C—H?I contacts between the (dppmO₂H⁺)_x methyl­ene‐group H atoms and the terminal I atoms of the triiodide anions, which display crystallographic inversion symmetry.