chromyl fluoride

Chromyl fluoride is an inorganic compound with the formula . It is a violet-red colored crystalline solid that melts to an orange-red liquid.Gard, G. L. (1986) "Chromium Difluoride Dioxide (Chromyl Fluoride)," '' Inorg. Synth.'', 24, 67-69, .

Structure

The liquid and gaseous have a tetrahedral geometry with C_2v symmetry, much like chromyl chloride. Chromyl fluoride dimerizes via fluoride bridges (as ) 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.

History and preparation

Pure chromyl fluoride was first isolated in 1952 as reported by Alfred Engelbrecht and Aristid von Grosse.Engelbrecht, A.; von Grosse, A. (1952) "Pure Chromyl Fluoride," '' J. Am. Chem. Soc.'' 74(''21''), 5262–5264, . It was first observed as red vapor in the early 19th century upon heating a mixture of fluorspar (), chromates, and sulfuric acid. These red vapors were initially thought to be , although some chemists assumed a structure analogous to . 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 by treating chromyl chloride with elemental fluorine.von Wartenberg, H. (1941) "Über höhere Chromfluoride (, und )" bout higher chromium fluorides (, and ) '' Z. Anorg. Allg. Chem.'' n German 247(''1-2''), 135–146, . Another attempt was made by Wiechert, who treated HF with dichromate, yielding impure liquid at −40 °C.

Engelbrecht and von Grosse's synthesis of , and most successive syntheses, involve treating chromium trioxide with a fluorinating agent:
:
The reaction is reversible, as water will readily hydrolyze back to .

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:
:
:
: (M = Mo, W)

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 . They reported 100% yield when the reactions were conducted at 120 °C. As expected from the relative reactivities of and , the molybdenum reaction proceeded more readily than did the tungsten.

Reactions

Chromyl fluoride is a strong oxidizing agent capable of converting hydrocarbons to ketones and carboxylic acids. It can also be used as a reagent in the preparation of other chromyl compounds. Like some other fluoride compounds, reacts with glass and quartz, so silicon-free plastics or metal containers are required for handling the compound. Its oxidizing power in inorganic systems has also been explored.Brown, S. D.; Green, P.J.; Gard, G.L. (1975) "The Chemistry of Chromyl Fluoride III: Reactions with Inorganic Systems," '' J. Fluorine Chem.'' 5(''3''), 203-219, . Chromyl fluoride can exchange fluorine atoms with metal oxides.

:

where M is a metal. Chromyl fluoride also converts the oxides of boron and silicon to their fluorides.

Chromyl fluoride reacts with alkali and alkaline earth metal fluorides in perfluoroheptane (solvent) to produce orange-colored tetrafluorodioxochromates(VI):
:
Chromyl fluoride also reacts with Lewis acids, drawing carboxylate ligands from organic acid anhydrides and producing an acyl fluoride byproduct:
:
Chromyl fluoride forms adducts with weak Lewis bases NO, , and .

Chromium oxytetrafluoride is prepared by fluorination of chromyl fluoride with krypton difluoride:
:

References

{{Chromium compounds

Chromium(VI) compounds
Chromium–halogen compounds
Oxidizing agents
Oxyfluorides
Chromium–oxygen compounds