# Difluorophosphate

Difluorophosphate or difluorodioxophosphate or phosphorodifluoridate is an anion with formula PO$_2$F−2. It has a single negative charge and resembles perchlorate (ClO−4) and monofluorosulfonate (SO$_3$F$^−$) 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

Ammonium difluorophosphate ([NH$_4$]PO$_2$F$_2$) 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 metal chlorides can react with dry difluorophosphoric acid to form alkali metal salts.
    NaCl + HPO$_2$F$_2$ → NaPO$_2$F$_2$ + HCl(g)
Fluorination of dichlorophosphates can produce difluorophosphates. Another method is fluorination of phosphates or polyphosphates.
Trimethylsilyl difluorophosphate ((CH$_3$)$_3$Si−O−P(=O)F$_2$) reacts with metal chlorides to give difluorophosphates.
The anhydride of difluorophosphoric acid (HPO$_2$F$_2$), phosphoryl difluoride oxide (P$_2$O$_3$F$_4$) reacts with oxides such as UO$_3$ to yield difuorophosphates. Phosphoryl difluoride oxide also reacts with alkali metal fluorides to yield difluorophosphates.

## Properties

The difluorophosphate ion in ammonium difluorophosphate and potassium difluorophosphate has these interatomic dimensions:
| Difluorophosphate salt | P–O length | P–F length | O–P–O angle | F–P–O angle | F–P–F angle |
| Ammonium difluorophosphate | 1.457 Å | 1.541 Å | 118.7° | 109.4° | 98.6° |
| Potassium difluorophosphate | 1.470 Å | 1.575 Å | 122.4° | 108.6° | 97.1° |
Hydrogen bonding from ammonium ion to oxygen atoms causes a change to the difluorophosphate ion in the ammonium salt.
On heating the salts that are not of alkali metals or alkaline earth metals, difluorophosphates decompose firstly by giving off POF$_3$ forming a monofluorophosphate (PO$_3$F$^{2−}$) compound, and then this in turn decomposes to an orthophosphate PO3−4 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 •PO$_2$F$^−$, •PO$_3$F$^−$ and •PO$_2$F$_2$.

## Compounds

| Formula | Name | Structure | Infrared spectrum | Melting point | Comments |
| LiPO$_2$F$_2$ | Lithium difluorophosphate | | | 360 °C | |
| Be(PO$_2$F$_2$)$_2$ | Beryllium difluorophosphate | | | >400 °C d | prepared from BeCl$_2$ and acid |
| CH$_3$CH$_2$−O−P(=O)F$_2$ | Ethyl difluorophosphate | | | | |
| [NH$_4$]$^+$PO$_2$F−2 | Ammonium difluorophosphate | orthorhombic: a = 8.13 Å, b = 6.43 Å, c = 7·86 Å, Z = 4 space group Pnma | P–F stretching 842 and 860 cm$^{−1}$; P–O stretching 1138 and 1292 cm$^{−1}$ | 213 °C | |
| [NO$_2$]$^+$PO$_2$F−2 | Nitronium difluorophosphate | | 515, 530, 550, 560, 575, 845, 880, 1145, 1300, 2390, 3760 cm$^{−1}$ | | nitronium formed from anhydride and N$_2$O$_5$ |
| [NO]$^+$PO$_2$F−2 | Nitrosonium difluorophosphate | | 500, 840, 880, 1130, 1272, 1315, 2278 cm$^{−1}$ | | nitrosonium formed from anhydride and N$_2$O$_3$ |
| NaPO$_2$F$_2$ | Sodium difluorophosphate | | | 210 °C | |
| Mg(PO$_2$F$_2$)$_2$ | Magnesium difluorophosphate | | | 200 °C | |
| [NH$_4$]$^+$Mg$^{2+}$(PO$_2$F−2)$_3$ | Ammonium magnesium difluorophosphate | Cmcm a=5.411 b=15.20 c=12.68 | | | |
| Al(PO$_2$F$_2$)$_3$ | Aluminium difluorophosphate | polymeric | 505, 541, 582, 642, 918, 971, 1200, 1290 cm$^{−1}$ (with 355 cm$^{−1}$ impurity) | | formed from Al(CH$_2$CH$_3$)$_3$ and acid; colourless insoluble powder |
| Si(−O−P(=O)F$_2$)$_4$ | Silicon(IV) difluorophosphate | | | | formed from SiCl$_4$ and anhydride |
| (CH$_3$)$_3$Si−O−P(=O)F$_2$ | Trimethylsilyl difluorophosphate | | | | formed from anhydride and [(CH$_3$)$_3$Si]$_2$O |
| KPO$_2$F$_2$ | Potassium difluorophosphate | orthorhombic: a = 8.03 Å, b = 6.205 Å, c = 7.633 Å, Z = 4, V=380.9 Å$^3$, density = 2.44 g/cm$^3$ | 510, 525, 570, 835, 880, 1145, 1320, 1340 cm$^{−1}$ | 263 °C | colourless elongated prisms |
| (K$^+$)$_4$(PO$_2$F−2)$_2$(S$_2$O2−7) | Tetrapotassium difluorophosphate pyrosulfate | C2/c: a = 13.00 Å, b = 7.543 Å, c = 19.01 Å, β = 130.07°, Z = 4 | | | |
| Ca(PO$_2$F$_2$)$_2$·CH$_3$COOCH$_2$CH$_3$ | Calcium difluorophosphate - ethyl acetate 1:1 solvate | | | | |
| Ca(PO$_2$F$_2$)$_2$ | Calcium difluorophosphate | | | >345 °C d | |
| [VO$_2$]$^+$PO$_2$F−2 | Pervanadyl difluorophosphate | | | | |
| CrO$_2$(PO$_2$F$_2$)$_2$ | Chromyl difluorophosphate | | | | formed from anhydride; red-brown |
| Cr(PO$_2$F$_2$)$_3$ | Chromium(III) difluorophosphate | | 320, 385, 490, 575, 905, 955, 1165, 1255 cm$^{−1}$ | | formed from excess anhydride, green |
| Mn(CO)$_5$PO$_2$F$_2$ | | | | 184 °C | |
| HMn(PO$_2$F$_2$)$_3$ | | | | | dissolve manganese in acid; white |
| [NH+4](Mn$^{2+}$)$_3$(PO$_2$F−2)(PO$_3$F$^{2−}$)$_2$(F$^−$)$_2$ | | | | | |
| Fe(PO$_2$F$_2$)$_2$ | Iron(II) difluorophosphate | | 463, 496, 668 (weak), 869 (double), 1139, 1290 cm$^{−1}$ | 180 °C d | colour blue green, hygroscopic, melts 250 °C, above 300 °C starts decomposing to Fe$_3$(PO$_4$)$_2$ |
| Fe(PO$_2$F$_2$)$_3$ | Iron(III) difluorophosphate | | 262, 493, 528, 570, 914, 965, 1173, 1242 cm$^{−1}$ | >400 °C | decomposes at 230 °C yielding FeF$_3$; dissolve iron in acid in presence of oxygen |
| K$^+$(Fe$^{2+}$)$_3$(PO$_2$F−2)(PO$_3$F$^{2−}$)$_2$(F$^−$)$_2$ | | | | | |
| Co(PO$_2$F$_2$)$_2$ | Cobalt(II) difluorophosphate | | | 173 °C | prepared from CoCl$_2$ and acid; pink or blue; blue formed by heating pink to 140 °C |
| HCo(PO$_2$F$_2$)$_3$ | | | | | dissolve cobalt in acid; red-purple |
| Co(PO$_2$F$_2$)$_2$·2CH$_3$CN | Cobalt(II) difluorophosphate - methyl cyanide solvate 1:2 | orthorhombic: a = 9.227 Å, b = 13.871 Å, c = 9.471 Å, V = 1212 Å$^3$, Z = 4, density = 1.88 g/cm$^3$ | | | treat HCo(PO$_2$F$_2$)$_3$ with CH$_3$CN for a few weeks; red crystals |
| [NH+4](Co$^{2+}$)$_3$(PO$_2$F−2)(PO$_3$F$^{2−}$)$_2$(F$^−$)$_2$ | | | | | |
| Ni(PO$_2$F$_2$)$_2$ | Nickel(II) difluorophosphate | | | 255 °C d | slowly prepared from NiCl$_2$ and acid; yellow |
| HNi(PO$_2$F$_2$)$_3$ | | | | | dissolve nickel in acid; yellow |
| Cu(PO$_2$F$_2$)$_2$ | Copper(II) difluorophosphate | orthorhombic Fddd: a = 10.134 Å, b = 24.49 Å, c = 34.06 Å, Z = 48, V = 8454.3 Å$^3$, density = 2.50 g/cm$^3$ | | 265 °C d | pale blue needles |
| CuI(xantphos)$_2$(μ-PO$_2$F$_2$) | | polymeric; monoclinic: a = 12.435 Å, b = 10.887 Å, c = 25.682 Å, β = 100.220°, V = 3421 Å$^3$ | | | colourless |
| Zn(PO$_2$F$_2$)$_2$ | Zinc(II) difluorophosphate | | | c. 25 °C? | glassy |
| H$_2$[Zn(PO$_2$F$_2$)$_4$] | Tetra(difluorophosphato)zincic(II) acid | | | | |
| Ga(PO$_2$F$_2$)$_3$ | Gallium(III) difluorophosphate | | | | |
| [(CH$_3$)$_2$GaPO$_2$F$_2$]$_2$ | Dimethylgallium(III) difluorophosphate | dimeric | 380, 492, 520, 551, 616, 709, 750, 899, 949, 1171, 1218, 1262, 1295, 1404, 2922, 2982 cm$^{−1}$ | | |
| RbPO$_2$F$_2$ | Rubidium difluorophosphate | orthorhombic: a = 8.15 Å, b = 6.45 Å, c = 7.79 Å, Z = 4, V = 409.5 Å$^3$ density = 3.02 g/cm$^3$ | P–F stretching 827 and 946 cm$^{−1}$; P–O stretching 1145 and 1320 cm$^{−1}$ | 160 °C | white |
| Sr(PO$_2$F$_2$)$_2$ | Strontium difluorophosphate | | | 250 °C d | prepared from SrCl$_2$ and acid |
| [NH$_4$]Sr(PO$_2$F$_2$)$_3$ | Ammonium strontium difluorophosphate | Triclinic P1 a=7.370 b=11.054 c=13.645 α=88.861 β=87.435° γ=89.323° | | | |
| AgPO$_2$F$_2$ | Silver(I) difluorophosphate | | | | |
| Ag$_9$(PO$_2$F$_2$)$_{14}$ | | | | | |
| Ag(1-methyl-2-alkylthiomethyl-1H-benzimidazole)PO$_2$F$_2$ | | | | | |
| Ag(2,6-bis-[(2-methylthiophenyl)-2-azaethenyl]pyridine)PO$_2$F$_2$ | | Triclinic P1: a = 7.687 Å, b = 10.740 Å, c = 13.568 Å, α = 99.52°, β = 96.83°, γ = 99.83°, Z = 2, V = 1076 Å$^3$, density = 1.81 g/cm$^3$ | | | |
| Ag(4,4′-dicyanodiphenylacetylene)PO$_2$F$_2$ | | | | | |
| Cd(PO$_2$F$_2$)$_2$ | Cadmium(II) difluorophosphate | | | 245 °C d | |
| In(PO$_2$F$_2$)$_3$ | Indium(III) difluorophosphate | | 269, 492, 528, 567, 910, 962, 1179, 1269 cm$^{−1}$ | | white, decomposes at 260 °C yielding InF$_3$ |
| [(CH$_3$)$_2$InPO$_2$F$_2$]$_2$ | Dimethylindium(III) difluorophosphate | dimeric | 373, 490, 500, 535, 559, 735, 878, 925, 1128, 1179, 1275, 1435, 2928, 3000 cm$^{−1}$ | | |
| SnCl$_2$(PO$_2$F$_2$)$_2$ | Tin(IV) dichloride difluorophosphate | | | | |
| (CH$_3$)$_2$Sn(PO$_2$F$_2$)$_2$ | Dimethyltin(IV) difluorophosphate | | | 204 °C d | prepared from (CH$_3$)$_2$SnCl$_2$ and acid; yellow |
| (CH$_3$CH$_2$)$_2$Sn(PO$_2$F$_2$)$_2$ | Diethyltin(IV) difluorophosphate | | | 262 °C d | prepared from (CH$_3$CH$_2$)$_2$SnCl$_2$ and acid; yellow |
| (CH$_3$CH$_2$CH$_2$)$_2$Sn(PO$_2$F$_2$)$_2$ | Dipropyltin(IV) difluorophosphate | | | 245 °C d | prepared from (CH$_3$CH$_2$CH$_2$)$_2$SnCl$_2$ and acid; yellow |
| (CH$_3$(CH$_2$)$_3$)$_2$Sn(PO$_2$F$_2$)$_2$ | Dibutyltin(IV) difluorophosphate | | | 235 °C d | prepared from (CH$_3$(CH$_2$)$_7$)$_2$SnCl$_2$ and acid; yellow |
| (CH$_3$(CH$_2$)$_7$)$_2$Sn(PO$_2$F$_2$)$_2$ | Dioctyltin(IV) difluorophosphate | | | 114 °C | prepared from (CH$_3$(CH$_2$)$_7$)$_2$SnCl$_2$ and acid; yellow |
| SbCl$_4$PO$_2$F$_2$ | Antimony(V) tetrachloride difluorophosphate | | | | |
| SbF$_4$PO$_2$F$_2$ | Antimony(V) tetrafluoride difluorophosphate | | | | |
| (2,2-dipyradyl)2Re(CO)$_2$PO$_2$F$_2$ | | | | | |
| Au[bis(triphenylphosphine sulfide-S)]PO$_2$F$_2$ | | | | | |
| IO$_2$PO$_2$F$_2$ | | | Raman: 130, 163, 191, 219, 295, 323, 329, 378, 637, 713, 737, 781, 799, 839, 918, 1163 cm$^{−1}$ | | yellowish colour, produced from IO$_3$, decomposed by water |
| IO$_3$PO$_2$F$_2$ | | | Raman: 217, 247, 269, 305, 343, 367, 395, 473, 569, 643, 671, 717, 797, 891, 1123 cm$^{−1}$ | | yellowish colour, produced from H$_5$IO$_6$, decomposed by water |
| FXePO$_2$F$_2$ | Xenon(II) fluoride difluorophosphate | | | | |
| Xe(PO$_2$F$_2$)$_2$ | Xenon(II) difluorophosphate | | | | |
| CsPO$_2$F$_2$ | Caesium difluorophosphate | orthorhombic: a = 8.437 Å, b = 6.796 Å, c = 8.06 Å, Z = 4, V = 462.1 Å$^3$, density = 3.36 g/cm$^3$ | | 286 °C | |
| (Cs$^+$)$_2$(Fe$^{3+}$)$_2$(PO$_2$F−2)(PO$_3$F$^{2−}$)$_2$(F$^−$)$_3$ | | | | | |
| Ba(PO$_2$F$_2$)$_2$ | Barium difluorophosphate | orthorhombic I42d a =10.4935 b =10.4935 c =26.030 | | >400 °C | |
| [NH$_4$]$_2$Ba(PO$_2$F$_2$)$_4$ | Diammonium barium difluorophosphate | P2/n a=14.285 b=5.472 c=19.474 β=97.607° | | | |
| Re(CO)$_5$PO$_2$F$_2$ | | | | | |
| Hg(PO$_2$F$_2$)$_2$ | Mercury(II) difluorophosphate | | | | |
| Hg$_2$(PO$_2$F$_2$)$_2$ | Mercury(I) difluorophosphate or di(difluorophosphato)dimercurane | Raman: 220 cm$^{−1}$ | | produced from anhydride | |
| TlPO$_2$F$_2$ | Thallium(I) difluorophosphate | | | | produced from anhydride, or acid on TlCl |
| [(CH$_3$)$_2$TlPO$_2$F$_2$]$_2$ | Dimethylthallium(III) difluorophosphate | dimeric | 360, 374, 500, 505, 520, 559, 850, 880, 1120, 1140, 1195, 1250, 1285, 2932, 3020 cm$^{−1}$ | | |
| Pb(PO$_2$F$_2$)$_2$ | Lead(II) difluorophosphate | | | 189 °C d | |
| UO$_2$(PO$_2$F$_2$)$_2$ | Uranyl difluorophosphate | | 260, 498, 854, 924, 980, 1124 cm$^{−1}$ | | IR spectrum due to UO2+2 |
| [(CH$_3$CH$_2$)$_4$N]$^+$PO$_2$F−2 | Tetraethylammonium difluorophosphate | | | | |
| | 1-ethyl-3-methylimidazolium difluorophosphate | | | | ionic liquid |
| | 1-butyl-3-methylimidazolium difluorophosphate | | | | ionic liquid |
| | 1-butyl-1-methylpyrrolidinium difluorophosphate | | | | ionic liquid |
| | 1-butyl-1-methylpiperidinium difluorophosphate | | | | ionic liquid |
| | di(3,3′,4,4′-tetramethyl-2,2′,5,5′-tetraselenafulvalenium)difluorophosphate | | | | Transitions to a metallic state below 137 K (−136 °C) |
| | 1,4-diphenyl-3,5-enanilo-4,5-dihydro-1,2,4-triazole (nitron) | monoclinic P2$_1$/n: a = 7.3811 Å, b = 14.9963 Å, c = 16.922 Å, β = 102.138°, V = 1361.2 Å$^3$, Z = 4 | | | insoluble; yellow-brown |
| Strychnine PO$_2$F$_2$ | | | | | |
| Cocaine PO$_2$F$_2$ | | | | | |
| Brucine PO$_2$F$_2$ | | | | | |
| Morphine PO$_2$F$_2$ | | | | | |
| [N(CH$_3$)$_4$]$^+$PO$_2$F−2 | Tetramethylammonium difluorophosphate | | | | |
| H[B(PO$_2$F$_2$)$_4$] | Tetra(difluorophosphato)boric acid | | 469, 502, 552, 647, 836, 940, 994, 1093, 1348, 1567 cm$^{−1}$ | | formed from BBr$_3$ and acid; liquid |
| Li[B(PO$_2$F$_2$)$_4$] | Lithium tetra(difluorophosphato)borate | monoclinic P2$_1$/c: a=7.9074 Å, b = 14.00602 Å, c = 13.7851 Å, β = 121.913°, Z = 4 | 479, 502, 568, 833, 945, 1002, 1080, 1334 cm$^{−1}$ | | formed from HB(PO$_2$F$_2$)$_4$ and butyllithium; colourless |
| [HS(CH$_3$)$_2$]$^+$[B(PO$_2$F$_2$)$_4$]$^−$ | Dimethylsulfonium tetra(difluorophosphato)borate | | 472, 511, 555, 648, 832, 933, 993, 1082, 1337, 1436, 2851, 2921, 3042 cm$^{−1}$ | | formed from BH$_3$·S(CH$_3$)$_2$ and acid; ionic liquid |
| [Li((CH$_3$CH$_2$)$_2$O)$^+$]$_3$[Al(PO$_2$F$_2$)$_6$]$^−$ | (Diethyl ether)lithium hexa(difluorophosphato)aluminate | trigonal R3: a = 17.4058 Å, b = 17.4058 Å, c = 21.4947 Å, γ = 120°, Z = 6 | 417, 503, 536, 624, 723, 891, 922, 964, 1174, 1204, 1283 cm$^{−1}$ | | formed from butyllithium and triethylaluminium and the acid; white |
| K$_2$CrO$_2$(PO$_2$F$_2$)$_4$ | | | 305, 370, 485, 550, 870, 920, 1050, 1130, 1250 cm$^{−1}$ | 145 °C d | formed from anhydride and K$_2$CrO$_4$; brown |
| Na$_2$MoO$_2$(PO$_2$F$_2$)$_4$ | | amorphous | 280, 490, 620, 880, 915, 950, 1020, 1070, 1140, 1280 cm$^{−1}$ | 125 °C d | formed from anhydride and K$_2$MoO$_4$; white |
| Na$_2$WO$_2$(PO$_2$F$_2$)$_4$ | | amorphous | 280, 474, 620, 930, 1030, 1130, 1230 cm$^{−1}$ | 109 °C d | formed from anhydride and K$_2$WO$_4$; white |

## Related substances
### Difluorphosphoric acid

Difluorophosphoric acid (HPO$_2$F$_2$) is one of the fluorophosphoric acids. It is produced when phosphoryl fluoride reacts with water:
    POF$_3$ + H$_2$O → HPO$_2$F$_2$ + HF
This in turn is hydrolysed more to give monofluorophosphoric acid (H$_2$PO$_3$F), and a trace of hexafluorophosphoric acid (HPF$_6$). HPO$_2$F$_2$ 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:
    POF$_3$ + H$_2$PO$_3$F → 2 HPO$_2$F$_2$
Difluorophosphoric acid can also be produced by fluorinating phosphorus oxychlorides. P$_2$O$_3$Cl$_4$ and POCl$_3$ react with hydrogen fluoride solution to yield HPO$_2$Cl$_2$ and then HPO$_2$F$_2$. Yet another way is to treat orthophosphate (PO3−4) with fluorosulfuric acid (HSO$_3$F).
Difluorphosphoric acid is a colorless liquid. It melts at −96.5 °C (−141.7 °F) and boils at 115.9 °C (240.6 °F). Its density at 25 °C is 1.583 g/cm$^3$.

### Phosphoryl difluoride oxide

Difluorophosphoric acid anhydride also known as phosphoryl difluoride oxide or diphosphoryl tetrafluoride (F$_2$(O=)P−O−P(=O)F$_2$ or P$_2$O$_3$F$_4$) is an anhydride of difluorphosphoric acid. It crystallises in the orthorhombic system, with space group Pcca and Z = 4. P$_2$O$_3$F$_4$ can be made by refluxing difluorophosphoric acid with phosphorus pentoxide. P$_2$O$_3$F$_4$ 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 PF$_5$ and AsF$_5$. 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 KPO$_2$F$_2$·2AsF$_5$, KPO$_2$F$_2$·AsF$_5$, KPO$_2$F$_2$·2PF$_5$ and KPO$_2$F$_2$·PF$_5$.
Amines can react with phosphoryl fluoride to make substances with a formula RR′N−P(=O)F$_2$. 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 (R−N=P(=O)F).