Difluorophosphate

Difluorophosphate or difluorodioxophosphate or phosphorodifluoridate is an anion with formula . It has a single negative charge and resembles perchlorate () and monofluorosulfonate (SO₃F⁻) in shape and compounds. These ions are isoelectronic, along with tetrafluoroaluminate, phosphate, orthosilicate, and sulfate. It forms a series of compounds. The ion is toxic to mammals as it causes blockage to iodine uptake in the thyroid. However it is degraded in the body over several hours.

Compounds containing difluorophosphate may have it as a simple uninegative ion, it may function as a difluorophosphato ligand where it is covalently bound to one or two metal atoms, or go on to form a networked solid. It may be covalently bound to a non metal or an organic moiety to make an ester or an amide.

Formation

The ammonium salt of difluorophosphate is formed from treating phosphorus pentoxide with ammonium fluoride. This was how the ion was first made by its discoverer, Willy Lange, in 1929.

Alkali chlorides can react with dry difluorophosphoric acid to form alkali metal salts.
:NaCl + HPO₂F₂ → NaPO₂F₂ + HCl()

Fluoridation of dichlorophosphates can produce difluorophosphates. Another method is fluorination of phosphates or polyphosphates.

Trimethylsilyl difluorophosphate reacts with metal chlorides to give difluorophosphates.

The anhydride phosphoryl difluoride oxide (P₂O₃F₄) reacts with oxides such as UO₃ to yield difuorophosphates. Phosphoryl difluoride oxide also reacts with alkali fluorides to yield difluorophosphates.

Properties

In ammonium difluorophosphate the difluorophosphate ion has these interatomic dimensions: P–O length 1.457 Å, P–F length 1.541 Å, O–P–O angle 118.7°, F–P–O 109.4° and F–P–F angle 98.6°. Hydrogen bonding from ammonium to oxygen atoms causes a change to the difluorophosphate ion in the ammonium salt. In potassium difluorophosphate the ion has dimensions: P–O length 1.470 Å, P–F length 1.575 Å, O–P–O angle 122.4°, F–P–O 108.6° and F–P–F angle 97.1°.

On heating the salts that are not of alkali or alkaline earths, difluorophosphates decompose firstly by giving off POF₃ forming a monofluorophosphate (PO₃F²⁻) compound, and then this in turn decomposes to an orthophosphate compound.

Difluorophosphate salts are normally soluble and stable in water. However, in acidic or alkaline conditions they can be hydrolyzed to monofluorophosphates and hydrofluoric acid. The caesium and potassium salts are the least soluble.

Irradiating potassium difluorophosphate with gamma rays can make the free radicals PO₂F^•−, PO₃F^•− and .

Compounds

Related substances

Difluorphosphoric acid

Difluorphosphoric acid (HPO₂F₂) is one of the fluorophosphoric acids. It is produced when phosphoryl fluoride reacts with water:
:POF₃ + H₂O → HPO₂F₂ + HF
This in turn is hydrolysed more to give monofluorophosphoric acid (H₂PO₃F), and a trace of hexafluorophosphoric acid (HPF₆). HPO₂F₂ also is produced when HF reacts with phosphorus pentoxide. Yet another method involves making difluorphosphoric acid as a side product of calcium fluoride being heated with damp phosphorus pentoxide. A method to make pure difluorphosphoric acid involves heating phosphoryl fluoride with monofluorophosphoric acid and separating the product by distillation:
:POF₃ + H₂PO₃F → 2HPO₂F₂

Difluorophosphoric acid can also be produced by fluorinating phosphorus oxychlorides. P₂O₃Cl₄ and POCl₃ react with hydrogen fluoride solution to yield HPO₂Cl₂ and then HPO₂F₂. Yet another way is to treat orthophosphate () with fluorosulfuric acid (HSO₃F).

Difluorphosphoric acid melts at −96.5 °C and boils at 115.9 °C. Its density at 25 °C is 1.583 g/cm³.

Phosphoryl difluoride oxide

Difluorophosphoric acid anhydride also known as phosphoryl difluoride oxide or diphosphoryl tetrafluoride (F₂OPOPOF₂ or P₂O₃F₄) is an anhydride of difluorphosphoric acid. It crystallises in the orthorhombic system, with space group ''Pcca'' and ''Z'' = 4. P₂O₃F₄ can be made by refluxing difluorophosphoric acid with phosphorus pentoxide. P₂O₃F₄ boils at 71 °C.

Substitution

In addition to the isoelectronic series, ions related by substituting fluorine or oxygen by other elements include monofluorophosphate, difluorothiophosphate, dichlorothiophosphate, dichlorophosphate, chlorofluorothiophosphate, chlorofluorophosphate, dibromophosphate, and bromofluorophosphate.

Adducts

Difluorophosphate can form adducts with PF₅ and AsF₅. In these the oxygen atoms form a donor-acceptor link between the P and As (or P) atoms, linking the difluorides to the pentafluorides. Example salts include KPO₂F₂ · 2AsF₅, KPO₂F₂·AsF₅, KPO₂F₂ · 2PF₅ and KPO₂F₂ · PF₅.

Amines can react with phosphoryl fluoride to make substances with a formula RR′N–POF₂. The amines shown to do this include ethylamine, isopropylamine, ''n''-butylamine, ''t''-butylamine, dimethylamine, and diethylamine. The monoamines can further react to yield an alkyliminophosphoric fluoride (RN=POF).

References

{{Reflist, 30em

Phosphorus oxyanions
Fluorophosphates
Anions