Silver(II) fluoride

Silver(II) fluoride is a chemical compound with the formula AgF₂. It is a rare example of a silver(II) compound. Silver is usually present in its +1 oxidation state. It is used as a fluorinating agent.

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Preparation

AgF₂ can be synthesized by fluorinating Ag₂O with elemental fluorine. Also, at 200 °C (473 K) elemental fluorine will react with AgF or AgCl to produce AgF₂.^[1][2]

As a strong fluorinating agent, AgF₂ should be stored in Teflon, a passivated metal container, or a quartz tube. It is light sensitive.

AgF₂ can be purchased from various suppliers, the demand being less than 100 kg/year. While laboratory experiments find use for AgF₂, it is too expensive for large scale industry use. In 1993, AgF₂ cost between 1000-1400 US dollars per kg.

Composition and structure

AgF₂ is a white crystalline powder, but it is usually black/brown due to impurities. The F/Ag ratio for most samples is < 2, typically approaching 1.75 due to contamination with Ag and oxides and carbon. ^[3]

For some time, it was doubted silver was actually in the 2+ oxidation state rather in some combination of states such as Ag^I[Ag^IIIF₄], which would be similar to silver oxide. Neutron diffraction studies, however, confirmed its description as silver(II). The Ag^I[Ag^IIIF₄] was found to be present at high temperatures, but it was unstable with respect to AgF₂.^[4]

In the gas phase, AgF₂ is believed to have D_∞h symmetry.

Approximately 14 kcal/mol (59 kJ/mol) separate the ground and first states. The compound is paramagnetic, but it becomes ferromagnetic at temperatures below −110 °C (163 K).

Uses

AgF₂ is a strong fluorinating and oxidation agent. Illustrative applications are listed below.

• Fluoronation and preparation of organic perfluorocompounds ^[5]

This type of reaction can occur in three different ways (here Z refers to any element or group attached to carbon, X is a halogen):

1) CZ₃H + 2 AgF₂ → CZ₃F +HF + 2 AgF

2) CZ₃X + 2AgF₂ → CZ₃F +X₂ + 2 AgF

3) Z₂C=CZ₂ + 2 AgF₂ → Z₂CFCFZ₂ + 2 AgF

Similar transformations can also be effected using other high valence metallic fluorides such as CoF₃, MnF₃, CeF₄, and PbF₄.

• Fluorinations of aromatic compounds is readily achieved, but selective monofluorinations are more difficult:^[6]

C₆H₆ + 2 AgF₂ → C₆H₅F + 2 AgF + HF

• Oxidation of xenon ^[7]. This reaction, which can be explosive, proceeds in anhydrous HF solutions producing XeF₂.

• AgF₂ oxidizes carbon monoxide to carbonyl fluoride.

References

1. ^  Priest, H. F. “Anhydrous Metal Fluorides” Inorganic Syntheses McGraw-Hill: New York, 1950; Vol. 3, pages 171-183.
2. ^  Encyclopedia of Chemical Technology. Kirk-Othermer. Vol.11, 4th Ed. (1991)
3. ^  J.T. Wolan, G.B. Hoflund. "Surface Characterization Study of AgF and AgF₂ Powders Using XPS and ISS," Applied Surface Science. 125, (1998).
4. ^  Hans-Christian Miller, Axel Schultz, and Magdolna Hargittai. "Structure and Bonding in Silver Halides: A Quantum...X=F, Cl, Br, I," Journal of the American Chemical Society 127(22), (2005).
5. ^  Rausch, D.; Davis, r.; Osborne, D. W. "The Addition of Fluorine to Halogenated Olefins by Means of Metal Fluorides," Journal of Organic Chemistry volume 28, pp. 494-497, Jul. (1962).
6. ^  Zweig, A.; Fischer, R. G.; Lancaster, J. "New Methods for Selective Monofluorination of Aromatics Using Silver Difluoride," Journal of Organic Chemistry volume 45, (1980).
7. ^  Levec, J.; Slivnik, J.; Zemva, B. "On the Reaction Between Xenon and Fluorine," Journal of Inorganic Nuclear Chemistry Volume 36, (1974).