Chromyl fluoride

Chromyl fluoride is an inorganic compound with the formula CrO₂F₂. It is a violet-red colored crystalline solid that melts to an orange-red liquid.

The liquid and gaseous CrO₂F₂ have a tetrahedral geometry with C_2v symmetry, much like chromyl chloride. Chromyl fluoride dimerizes via fluoride bridges (as O₂Cr(μ-F)₄CrO₂) in the solid state, crystallizing in the P2₁/c space group with Z = 4. The Cr=O bond lengths are about 157 pm, and the Cr–F bond lengths are 181.7, 186.7, and 209.4 pm. Chromium resides in a distorted octahedral position with a coordination number of 6.

Pure chromyl fluoride was first isolated in 1952 as reported by Alfred Engelbrecht and Aristid von Grosse. It was first observed as red vapor in the early 19th century upon heating a mixture of fluorspar (CaF₂), chromates, and sulfuric acid. These red vapors were initially thought to be CrF₆, although some chemists assumed a CrO₂F₂ structure analogous to CrO₂Cl₂. The first moderately successful synthesis of chromyl fluoride was reported by Fredenhagen who examined the reaction of hydrogen fluoride with alkali chromates. A later attempt saw von Wartenberg prepare impure CrO₂F₂ by treating chromyl chloride with elemental fluorine. Another attempt was made by Wiechert, who treated HF with dichromate, yielding impure liquid CrO₂F₂ at −40 °C.

Engelbrecht and von Grosse's synthesis of CrO₂F₂, and most successive syntheses, involve treating chromium trioxide with a fluorinating agent:

The reaction is reversible, as water will readily hydrolyze CrO₂F₂ back to CrO₃.

The approach published by Georg Brauer in the Handbook of Preparative Inorganic Chemistry drew on von Wartenberg's approach of direct fluoridation:

Other methods include treatment with chlorine fluoride, carbonyl fluoride, or some metal hexafluorides:

The last method involving the fluorides of tungsten and molybdenum are reported by Green and Gard to be very simple and effective routes to large quantities of pure CrO₂F₂. They reported 100% yield when the reactions were conducted at 120 °C. As expected from the relative reactivities of MoF₆ and WF₆, the molybdenum reaction proceeded more readily than did the tungsten.