Arsenic trifluoride is a chemical compound of arsenic and fluorine with the chemical formula AsF<sub>3</sub>. It is a colorless liquid which reacts readily with water. Like other inorganic arsenic compounds, it is highly toxic.
It can be prepared by treating arsenic trioxide with hydrogen fluoride:
It has a pyramidal molecular structure in the gas phase which is also present in the solid. In the gas phase the As-F bond length is 170.6 pm and the F-As-F bond angle 96.2ð.
Arsenic trifluoride is used as a fluorinating agent for the conversion of non-metal chlorides to fluorides, in this respect it is less reactive than SbF<sub>3</sub>.
Salts containing AsF<sub>4</sub><sup><nowiki>−</nowiki></sup> anion can be prepared for example CsAsF<sub>4</sub>. the potassium salt KAs<sub>2</sub>F<sub>7</sub> prepared from KF and AsF<sub>3</sub> contains AsF<sub>4</sub><sup><nowiki>−</nowiki></sup> and AsF<sub>3</sub> molecules with evidence of interaction between the AsF<sub>3</sub> molecule and the anion.
AsF<sub>3</sub> reacts with SbF<sub>5</sub>. The product obtained could be described as the ionic compound AsF<sub>2</sub><sup>+</sup> SbF<sub>6</sub><sup><nowiki>−</nowiki></sup>. However, the authors conclude the formed product cannot be viewed only as an ionic compound nor entirely as the neutral adduct AsF<sub>3</sub>SbF<sub>5</sub>. The crystal structure displays characteristics of both an ionic pair, and a neutral adduct, taking the middle ground in between both models.