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Cobalt(III) fluoride (also known as cobalt trifluoride) is the inorganic compound with the formula CoF3. The compound exists in both hydrated and anhydrous forms, the latter being a hygroscopic brown solid.[2] It is used as a fluorinating agent in organofluorine synthesis.[3]
The related cobalt(III) chloride is also known but is extremely unstable.[4] Cobalt(III) bromide and cobalt(III) iodide have not been synthesized.
Anhydrous cobalt trifluoride crystallizes in the rhombohedral group, specifically according to the aluminium trifluoride motif, with a = 527.9 pm and α = 56.97°. Each cobalt centre is bound to six fluorine atoms in octahedral geometry, with Co–F distances of 189 pm. Each fluoride is a doubly bridging ligand.[2]
A hydrate CoF3·3.5H2O is known, which is conjectured to be better described as [CoF3(H2O)3]·0.5H2O.[2]
There is a report of a hydrate CoF3·3.5H2O, isomorphic to AlF3·3H2O.[2]
Cobalt trifluoride can be prepared in the laboratory by treating cobalt(II) chloride (CoCl2) with fluorine gas at 250 °C:[2][5]
In this redox reaction, the reduction of fluorine to fluoride ions drives the oxidation of both cobalt(II) cations and chloride anions to cobalt(III) ions and chlorine gas, respectively. Treatment of cobalt(II) chloride with chlorine trifluoride (ClF3) or bromine trifluoride (BrF3) also yield cobalt trifluoride, as does the direct fluoridation of cobalt(II) oxide or cobalt(II) fluoride:[2]
The other stable oxide of cobalt, cobalt(II,III) oxide (Co3O4), can be sequentially treated with hydrogen fluoride and then fluorine to produce first cobalt(II) fluoride and cobalt oxyfluoride and then cobalt(III) fluoride, with the overall stoichiometry:[6]
This process reduces the need for expensive and difficult-to-handle fluorine gas.[6]
CoF3 decomposes upon contact with water to give oxygen:
It reacts with fluoride salts to give the hexafluorocobaltate(III) anion (CoF3−6), which features a high-spin, octahedral cobalt(III) center.
Synthesis of organofluorine compounds can be undertaken by direct reaction with fluorine, but this approach can result in fragmentation of the target hydrocarbon.[6]CoF3 is an alternative fluorinating agent that is still powerful, but milder than direct fluorination with fluorine.[6][7] Used as slurry, CoF3 converts hydrocarbons to the perfluorocarbons:[7]
CoF₂は副生成物である。
ブタンの三フッ化コバルトフッ素化の研究では、 51種類以上のポリフッ素化およびパーフルオロ化ブタンの混合物と、いくつかの置換メチルプロパンが生成されることが示されました。[ 8 ]シクロペンタン のフッ素化でも同様に、 5個（C5H5F5）から10個（C5F10）のフッ素置換を持つ生成物の混合物が得られました。 [ 9 ]選択性の低い多数の生成物が生成されるため、三フッ化コバルト フッ素化の合成上の有用性は大きく制限されます。[ 3 ]関連試薬であるテトラフルオロコバルト酸カリウム(III)（KCoF4 ）は、より選択性の高い代替手段です。[ 10 ]
気相では、CoF 3 は基底状態で平面であり、3 回回転軸 (点群 D 3h ) を持つことが計算されています。Co 3+イオンは 3d 6 5 Dの基底状態を持ちます。フッ化物配位子はこの状態をエネルギー順に5 A'、5 E"、および5 E' の状態に分割します。最初のエネルギー差は小さく、5 E" 状態はヤーン・テラー効果を受けるため、基底状態を確実にするためにはこの効果を考慮する必要があります。エネルギーの低下は小さく、エネルギー順序は変わりません。[ 11 ]この計算は、計算されたエネルギー面を使用してヤーン・テラー効果を扱った最初のものでした。