Silver Trifluoride: Preparation, Crystal Structure, Some Properties, and Comparison with AUF3

Article abstract of DOI:10.1021/ja00011a021

Red, diamagnetic AgF₃ is precipitated from anhydrous hydrogen fluoride (AHF) solutions of AgF₄^- salts by addition of fluoro acids (L) such as BF₃, PF₅, or AsF₅: AgF₄^-+ L → AgF₃ + LF^-. With additional AsF₅, silver(III) is reduced: AgF₃ + AsF_f → AgFAsF₆ + 1/2F₂. Such reduction does not occur with BF₃, and the latter acid is therefore preferred for the preparation of stoichiometric AgF₃. AgF₃ is thermodynamically unstable and, in contact with AHF, at ～20 °C, loses F₂ in less than 19 h according to the equation 3AgF₃ → Ag₃F₈ + 1/2F₂. The trifluoride is isostructural with AuF₃. To provide for meaningful comparisons, the structure of AuF₃ was redetermined. AgF₃ and AuF₃ were successfully refined in space group P6₁-D₆₂ (No. 178) by using the Rietveld method from time-of-flight neutron powder diffraction data from 100-mg samples contained in 2-mm capillary tubes. The 7762 observations for AgF₃ yielded a = 5.0782 (2) A?, c = 15.4524 (8) A?, and V= 345.10 (2) A?³, the reliability parameters for the structure being R_wp = 6.21 and R_p = 3.83%. From the 7646 observations for AuF₃, a = 5.1508 (1) A?, c= 16.2637 (7) A?, V= 373.68 (2) A?³3, and R_wp = 11.21 and R_p = 7.58%. The lilver or gold atom lies at the center of an elongated octahedron with two Ag-F(1) = 1.990 (3) A?, two Au-F(1) = 1.998 (2) A?, two Ag-F(2) = 1.863 (4) A?, and two Au-F(2) = 1.868 (3) A?, the approximately square, isodimensional AF₄ units being joined by symmetrical μ-fluoro bridges (two F(1) in cis relationship in the AF₄ unit) to form the 6₁ (or 6₅) helical chains where Ag-F(1)-Ag = 123.2 (2)° and Au-F(1)-Au = 119.3 (2)°. The ～5-A?³ smaller formula unit volume of AgF₃ compared with AuF₃ and the shorter z axis interatomic distance (Ag-F = 2.540 (4), Au-F = 2.756 (8) A?) are in accord with the tighter binding of the Ag(III) d-orbital electrons evident in the strong oxidizing properties of Ag(III). Interaction of AgF⁺ with AgF₄^- (1:1) in AHF yields maroon Ag^IIAg^IIIF₅. The latter interacts with AgF₃ to yield Ag^IIAg^III₂F₈, which is identical with the product of the decomposition of AgF₃ at 20 °C in AHF and with the material previously described^1,2 AgF₃. The magnetic susceptibility for Ag^IIAg^III₂F₈ obeys the Curie-Weiss law (4-280 K) with θ = -4.2 (5) ° and μ_eff = 1-924 (3) μ_B.