Phosphorus pentaiodide

Phosphorus pentaiodide is a hypothetical inorganic compound with formula PI5. 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 PI3*I2, but certainly not as [PI4]+I−...)".

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 [PI4]+I− (tetraiodophosphonium iodide), experimental and theoretical data refutes this claim.

Derivatives
Unlike the elusive PI5, the [PI4]+ cation (tetraiodophosphonium cation) is widely known. This cation is known with the anions tetraiodoaluminate [AlI4]−, hexafluoroarsenate [AsF6]−, hexafluoroantimonate [SbF6]− and tetraiodogallate [GaI4]−.