Phosphorus pentaiodide is a hypothetical inorganic compound with formula . The existence of this compound has been claimed intermittently since the early 1900s. The claim is disputed: "The pentaiodide does not exist (except perhaps as , but certainly not as ...)".
Phosphorus pentaiodide was reported to be a brown-black crystalline solid, melting at 41 ðC, produced by the reaction of lithium iodide and phosphorus pentachloride in methyl iodide, however, this claim is disputed and probably generated a mixture of phosphorus triiodide and iodine.
Although phosphorus pentaiodide has been claimed to exist in the form of (tetraiodophosphonium iodide), experimental and theoretical data refutes this claim.
Unlike the elusive , the cation (tetraiodophosphonium cation) is widely known. This cation is known to produce salts with the anions tetraiodoaluminate , hexafluoroarsenate , hexafluoroantimonate and tetraiodogallate .