SILVER TRIFLUORIDE - PREPARATION, CRYSTAL-STRUCTURE, SOME PROPERTIES, AND COMPARISON WITH AUF3

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 + AsF5 --> AgFAsF6 + 1/2 F2. 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 approximately 20-degrees-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 P6(1)22-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) angstrom, c = 15.4524 (8) angstrom, and V = 345.10 (2) angstrom-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) angstrom, c = 16.2637 (7) angstrom, V = 373.68 (2) angstrom-3, and R(wp) = 11.21 and R(p) = 7.58%. The silver or gold atom lies at the center of an elongated octahedron with two Ag-F(1) = 1.990 (3) angstrom, two Au-F(1) = 1.998 (2) angstrom, two Ag-F(2) = 1.863 (4) angstrom, and two Au-F(2) = 1.868 (3) angstrom, the approximately square, isodimensional AF4 units being joined by symmetrical mu-fluoro bridges (two F(1) in cis relationship in the AF4 unit) to form the 6(1) (or 6(5)) helical chains where Ag-F(1)-Ag = 123.2 (2)-degrees and Au-F(1)-Au = 119.3 (2)-degrees. The approximately 5-angstrom-3 smaller formula unit volume of AgF3 compared with AuF3 and the and the shorter z axis interatomic distance (Ag-F = 2.540 (4), Au-F = 2.756 (8) angstrom) 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(II)Ag(III)F5. The latter interacts with AgF3 to yield Ag(II)Ag(III)2F8, which is identical with the product of the decomposition of AgF3 at 20-degrees-C in AHF and with the material previously described1,2 as AgF3. The magnetic susceptibility for Ag(II)Ag(III)2F8 obeys the Curie-Weiss law (4-280 K) with theta = -4.2 (5)-degrees and mu-eff = 1.924 (3)-mu-B.