Thermodynamically unstable fluorides of nickel: NiF4 and NiF3 syntheses and some properties

Article abstract of DOI:10.1021/ja00145a013

F^- acceptors (BF₃, AsF₅, SbF₅, or BiF₅) added to solutions of NiF₆^2- salts in anhydrous hydrogen fluoride (aHF) below -65 °C precipitate the tan solid NiF₄. This solid, preserved at ≤-65 °C, is quantitatively converted, by 2 equiv of F^- donor (XeF₆ or KF) in aHF, to dissolved NiF₆^2-. Dry NiF₄ loses F₂ above -60 °C, the decomposition to nearly black NiF₃ becoming rapid at ～0 °C. When the dry NiF₄ is prepared from K₂NiF₆, inclusion of some K⁺ leads, on thermolysis at 0 °C, to a pyrochlore form of NiF₃ (P-NiF₃). P-NiF₃ contains K⁺ in the open channels, with K_xMiF₃, x ≈ 0.1. The nearly cubic P-NiF₃ unit cell is rhombohedral: a₀ = 9.933(3) A?, α = 91.01(3)°, V = 980 A?³, z = 16, with absent reflections coincident with those of the cubic space group Oh⁷-Fd3m, appropriate for pyrochlore. Decomposition of NiF₄ in aHF begins at -65 °C and is rapid at 0 °C, giving black rhombohedral NiF₃ (A-NiF₃) with a₀ = 5.168(2) A?, α = 55.46(3) A°, V = 87.3 A?³, z = 2. When the NiF₄ is made and decomposed at ～20 °C, with K⁺ present, a hexagonal tungsten bronze form of NiF₃ is precipated (H-NiF₃), with a₀ = 7.074(6) A?, c₀ = 7.193(6) A?, V = 312A?³, z = 6. R- and H-NiF₃ can also be made by mixing solutions of Ni²⁺ salts [e.g., Ni(AsF₆)₂] with NiF₆^2- salts (e.g. K₂NiF₆) in aHF. All forms of the trifluoride (R, H, and P) lose F₂ on warming (R > 39°, H > 72° and P > 138 °C) to yield NiF₂, but an intermediate red-brown phase is observed for R-NiF₃. R-NiF₃ at ～20 °C, oxidizes Xe to Xe(VI), perfluoropropene, C₃F₆, to perfluoropropane, C₃F₈, and solid LiCl with incandescence. H-NiF₃ and P-NiF₃ interact similarly but less energetically.