Cobalt(III) fluoride

Cobalt(III) fluoride is the inorganic compound with the formula CoF3. Hydrates are also known. The anhydrous compound is a hygroscopic brown solid. It is used to synthesize organofluorine compounds.

The related cobalt(III) chloride is also known but is extremely unstable. Cobalt(III) bromide and cobalt(III) iodide have not been synthesized.

Anhydrous
Anhydrous cobalt trifluoride crystallizes in the rhombohedral group, specifically according to the aluminium trifluoride motif, with a = 527.9 pm, α = 56.97°. Each cobalt atom is bound to six fluorine atoms in octahedral geometry, with Co–F distances of 189 pm. Each fluoride is a doubly bridging ligand.

Hydrates
A hydrate CoF3*3.5H2O is known. It is conjectured to be better described as [CoF3(H2O)3]*0.5H2O.

There is a report of an hydrate CoF3*3.5H2O, isomorphic to AlF3*3H2O.

Preparation
Cobalt trifluoride can be prepared in the laboratory by treating CoCl2 with fluorine at 250 °C:
 * CoCl2 +  3/2 F2  →    CoF3  +  Cl2

In this redox reaction, Co(2+) and Cl(-) are oxidized to Co(3+) and Cl2, respectively, while F2 is reduced to F(-). Cobalt(II) oxide (CoO) and cobalt(II) fluoride (CoF2) can also be converted to cobalt(III) fluoride using fluorine.

The compound can also be formed by treating CoCl2 with chlorine trifluoride ClF3 or bromine trifluoride BrF3.

Reactions
CoF3 decomposes upon contact with water to give oxygen:
 * 4 CoF3 +  2 H2O →  4 HF  +  4 CoF2  +  O2

It reacts with fluoride salts to give the anion [CoF6]3−, which is also features high-spin, octahedral cobalt(III) center.

Applications
CoF3 is a powerful fluorinating agent. Used as slurry, CoF3 converts hydrocarbons to the perfluorocarbons:
 * 2 CoF3 +  R-H  →  2 CoF2  +  R-F  +  HF

CoF2 is the byproduct.

Such reactions are sometimes accompanied by rearrangements or other reactions. The related reagent KCoF4 is more selective.

Gaseous CoF3
In the gas phase, CoF3 is calculated to be planar in its ground state, and has a 3-fold rotation axis (point group D3h). The Co(3+) ion has a ground state of 3d6 5D. The fluoride ligands split this state into, in energy order, 5A', 5E", and 5E' states. The first energy difference is small and the 5E" state is subject to the Jahn-Teller effect, so this effect needs to be considered to be sure of the ground state. The energy lowering is small and does not change the energy order. This calculation was the first treatment of the Jahn-Teller effect using calculated energy surfaces.