Chromyl fluoride

The liquid and gaseous {{chem2|CrO2F2}} have a tetrahedral geometry with C_2v symmetry, much like chromyl chloride.Hobbs, W. E. (1958) "Infrared Absorption Spectra of Chromyl Fluoride and Chromyl Chloride," J. Chem. Phys. 28(6), 1220-1222, {{doi|10.1063/1.1744372}}. Chromyl fluoride dimerizes via fluoride bridges (as {{chem2|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.Supeł, J.; Abram, U.; Hagenbach, A.; Seppelt, K. (2007) "Technetium Fluoride Trioxide, TcO₃F, Preparation and Properties." Inorg. Chem., 46(14), 5591–5595, {{doi|10.1021/ic070333y}}.

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, {{doi|10.1021/ja01141a007}}. It was first observed as red vapor in the early 19th century upon heating a mixture of fluorspar ({{chem2|CaF2}}), chromates, and sulfuric acid. These red vapors were initially thought to be {{chem2|CrF6}}, although some chemists assumed a {{chem2|CrO2F2}} structure analogous to {{chem2|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 {{chem2|CrO2F2}} by treating chromyl chloride with elemental fluorine.von Wartenberg, H. (1941) "Über höhere Chromfluoride ({{chem|CrF|4}}, {{chem|CrF|5}} und {{chem|CrO|2|F|2}})" [About higher chromium fluorides ({{chem|CrF|4}}, {{chem|CrF|5}} and {{chem|CrO|2|F|2}})], Z. Anorg. Allg. Chem. [in German], 247(1-2), 135–146, {{doi|10.1002/zaac.19412470112}}. Another attempt was made by Wiechert, who treated HF with dichromate, yielding impure liquid {{chem2|CrO2F2}} at −40 °C.

Engelbrecht and von Grosse's synthesis of {{chem2|CrO2F2}}, and most successive syntheses, involve treating chromium trioxide with a fluorinating agent:

:{{chem2|CrO3 + 2 HF → CrO2F2 + H2O}}

The reaction is reversible, as water will readily hydrolyze {{chem2|CrO2F2}} back to {{chem2|CrO3}}.

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

:{{chem2|CrO2Cl2 + F2 → CrO2F2 + Cl2}}

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

:{{chem2|CrO3 + 2 ClF → CrO2F2 + Cl2 + O2}}

:{{chem2|CrO3 + COF2 → CrO2F2 + CO2}}

:{{chem2|CrO3 + MF6 → CrO2F2 + MOF4}} (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 {{chem2|CrO2F2}}. They reported 100% yield when the reactions were conducted at 120 °C. As expected from the relative reactivities of {{chem2|MoF6}} and {{chem2|WF6}}, the molybdenum reaction proceeded more readily than did the tungsten.Green, P. J.; Gard, G. L. (1977) "Chemistry of Chromyl Fluoride. 5. New Preparative routes to CrO₂F₂," Inorg. Chem. 16(5), 1243–1245, {{doi|10.1021/ic50171a055}}.

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, {{chem2|CrO2F2}} 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, {{doi|10.1016/S0022-1139(00)82482-3}}. Chromyl fluoride can exchange fluorine atoms with metal oxides.

:{{chem2|CrO2F2 + MO → MF2 + CrO3}}

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

Chromyl fluoride reacts with alkali and alkaline earth metal fluorides in perfluoroheptane (solvent) to produce orange-colored tetrafluorodioxochromates(VI):

:{{chem2|CrO2F2 + 2 MF → (M+)2[CrO2F4](2-)}}

Chromyl fluoride also reacts with Lewis acids, drawing carboxylate ligands from organic acid anhydrides and producing an acyl fluoride byproduct:

:{{chem2|CrO2F2 + 2 (CF3CO)2O → (CF3COO)2CrO2 + 2 CF3COF}}

Chromyl fluoride forms adducts with weak Lewis bases NO, {{chem2|NO2}}, and sulfur dioxide.

Chromium oxytetrafluoride is prepared by fluorination of chromyl fluoride with krypton difluoride:{{cite journal |doi=10.1021/ic00233a013 |title=Synthesis and characterization of CrF4O, KrF2.CrF4O, and NO+CrF5O- |date=1986 |last1=Christe |first1=Karl O. |last2=Wilson |first2=William W. |last3=Bougon |first3=Roland A. |journal=Inorganic Chemistry |volume=25 |issue=13 |pages=2163–2169}}

:{{chem2|2 CrO2F2 + 2 KrF2 -> 2 CrOF4 + O2 + 2 Kr}}

{{Chromium compounds}}

Category:Chromium(VI) compounds

Category:Chromium–halogen compounds

Category:Oxidizing agents

Category:Oxyfluorides

Category:Chromium–oxygen compounds