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 ...)".
Claims
Phosphorus pentaiodide was reported to be a brown-black crystalline solid melting at 41 °C 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.
Derivatives
Unlike the elusive , the cation (tetraiodophosphonium cation) is widely known. This cation is known with the anions tetraiodoaluminate , hexafluoroarsenate , hexafluoroantimonate and tetraiodogallate .