Xenon Difluoride
Properties
| State | Solid (vapor pressure 4.5 mmHg at 25 °C; sublimes on heating at 1 atm) |
| Color | Colorless |
| Solubility | Reacts with water slowly to release O2 and HF; soluble in anhydrous HF |
| Melting Point | 128.6 °C |
About Xenon Difluoride
Xenon difluoride, XeF2, is a colorless crystalline molecular solid that broke a textbook rule when Howard Claassen's group at Argonne first made it in 1962, only months after Neil Bartlett's xenon hexafluoroplatinate proved noble gases were not actually inert. It remains the most thermally stable xenon compound and the only one most labs ever handle. The molecule is linear with an F-Xe-F angle of 180 degrees and a 3-center 4-electron bond down the F-Xe-F axis, with three lone pairs occupying the equatorial plane around the hypervalent xenon. Synthesis is straightforward by 1962 standards: heat xenon and fluorine to 400 °C in a nickel vessel, or photolyze them with a mercury lamp at room temperature, then condense the product as colorless crystals stable indefinitely in dry glass. As a fluorinating agent XeF2 is unusually clean. It converts alcohols to alkyl fluorides, takes aromatic rings to aryl fluorides, and gives vicinal difluorides on alkenes without the over-fluorination disasters that elemental F2 brings. The byproduct is just xenon gas. The semiconductor industry has built a major application around the same selectivity: XeF2 vapor etches silicon to volatile SiF4 with extreme selectivity over SiO2, Si3N4, photoresist, and metal masks, which is why it is the workhorse for releasing MEMS structures.
Where you'll encounter it
If you've ever read about how the cantilever beams in an accelerometer or the membrane of a pressure sensor got freed from the silicon substrate they were patterned on, you've encountered XeF2 etching even if the datasheet did not advertise it. In a MEMS fabrication line, XeF2 sits in a small steel cylinder feeding a vacuum chamber where wafers are exposed to short pulses of XeF2 vapor at room temperature, releasing patterned silicon microstructures while leaving the SiO2 mask layers intact — much gentler than plasma etching and the only viable release for many fragile cantilever, gear, and microfluidic geometries. Synthetic chemists doing fluorine-18 PET tracer prep use XeF2 (or its 18F-labeled analogs) as a clean late-stage fluorination reagent because the only byproduct is xenon gas that pumps away. A handful of teaching labs at well-equipped graduate institutions still order XeF2 from Sigma-Aldrich just to demonstrate noble-gas chemistry to inorganic students.
Common Uses
- Selective fluorinating agent for alkenes, aromatic rings, and alcohols in organic and pharmaceutical synthesis
- Isotropic dry-release etchant for silicon in MEMS, microfluidic, and accelerometer fabrication
- Late-stage fluorination reagent for PET imaging tracers and 18F-labeled compounds
- Precursor for xenon oxyfluoride and xenon fluorocation chemistry in main-group research
- Teaching demonstration of noble-gas hypervalent bonding in graduate inorganic curricula
- Research reagent for accessing high-oxidation-state main-group fluoride compounds
Safety Information
XeF2 is acutely toxic and water-reactive. Contact with moisture or organic material slowly releases hydrogen fluoride, which is itself a Category 1 acute toxin and causes deep dermal burns and systemic hypocalcemia. GHS: Acute Toxicity Inhalation Category 2, Skin Corrosion 1B, Eye Damage 1, Water-reactive. There is no specific OSHA PEL for XeF2; HF byproduct exposure is governed by the OSHA HF PEL of 3 ppm TWA. Store under strictly dry conditions in PFA or dry glass with PTFE valves, handle only inside a fume hood with calcium gluconate 2.5% gel kept within arm's reach for HF first aid, and wear neoprene gloves over nitrile, full face shield, and chemical apron for any open transfer.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.