Phosphonium iodide is a chemical compound with the formula . It is an example of a salt containing an unsubstituted

phosphonium In polyatomic cations with the chemical formula (where R is a hydrogen or an alkyl, aryl, or halide group). These cations have tetrahedral structures. The salts are generally colorless or take the color of the anions. Types of phosphonium c ...

cation (). Phosphonium iodide is commonly used as storage for

phosphine Phosphine ( IUPAC name: phosphane) is a colorless, flammable, highly toxic compound with the chemical formula , classed as a pnictogen hydride. Pure phosphine is odorless, but technical grade samples have a highly unpleasant odor like rotti ...

and as a reagent for substituting phosphorus into organic molecules.

Preparation

Phosphonium iodide is prepared by mixing

diphosphorus tetraiodide Diphosphorus tetraiodide is an orange crystalline solid with the formula P2I4. It has been used as a reducing agent in organic chemistry. It is a rare example of a compound with phosphorus in the +2 oxidation state, and can be classified as a su ...

() with elemental phosphorus and water at 80 °C and allowing the salt to sublime.

Properties

Structure

Its crystal structure has the

tetragonal In crystallography, the tetragonal crystal system is one of the 7 crystal systems. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a squar ...

space group P4/nmm, which is a distorted version of the NH₄Cl crystal structure; the unit cell has approximate dimensions 634×634×462 pm. The

hydrogen bonding In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing ...

in the system causes the cations to orient such that the hydrogen atoms point toward the anions.

Chemical

At 62 °C and atmospheric pressure, phosphonium iodide

sublimates Sublimation is the transition of a substance directly from the solid to the gas state, without passing through the liquid state. Sublimation is an endothermic process that occurs at temperatures and pressures below a substance's triple poi ...

and dissociates reversibly into phosphine and

hydrogen iodide Hydrogen iodide () is a diatomic molecule and hydrogen halide. Aqueous solutions of HI are known as hydroiodic acid or hydriodic acid, a strong acid. Hydrogen iodide and hydroiodic acid are, however, different in that the former is a gas under s ...

(HI). It oxidizes slowly in air to give iodine and

phosphorus oxide Phosphorus oxide can refer to: * Phosphorus pentoxide (phosphorus(V) oxide, phosphoric anhydride), P2O5 * Phosphorus trioxide (phosphorus(III) oxide, phosphorous anhydride), P2O3 * Phosphorus tetroxide, * Several other, less common, oxides of ph ...

s; it is

hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substance ...

and is

hydrolyzed Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolysis ...

into phosphine and HI: : Phosphine gas may be devolved from phosphonium iodide by mixing an aqueous solution with

potassium hydroxide Potassium hydroxide is an inorganic compound with the formula K OH, and is commonly called caustic potash. Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which expl ...

: : It reacts with elemental

iodine Iodine is a chemical element with the Symbol (chemistry), symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , ...

and

bromine Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table (halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simil ...

in a nonpolar solution to give phosphorus halides; for example: : Phosphonium iodide is a powerful substitution reagent in organic chemistry; for example, it can convert a

pyrilium Pyrylium is a cation (positive ion) with formula , consisting of a six-membered ring of five carbon atoms, each with one hydrogen atom, and one positively charged oxygen atom. The bonds in the ring are conjugated as in benzene, giving it an ...

into a phosphinine via substitution. In 1951, Glenn Halstead Brown found that PH₄I reacts with

acetyl chloride Acetyl chloride (CH3COCl) is an acyl chloride derived from acetic acid. It belongs to the class of organic compounds called acid halides. It is a colorless, corrosive, volatile liquid. Its formula is commonly abbreviated to AcCl. Synthesis On ...

to produce an unknown phosphine derivative, possibly .

References

{{reflist Inorganic phosphorus compounds Phosphonium compounds Iodides Reagents for organic chemistry