Abstract. S4N4.AsF 5, monoclinic, P2~/m, a = 5.593(1), b = 11.062(2), c = 7.682(1) A, fl = 109.10 (1) °, U = 449.2 (2) A 3, Dc = 2.62 Mg m -3 for Z = 2; R~ = 0.036 for 848 non-zero data. The structure of this compound is comparable to those of all the other 1:1 adducts of S4N 4, with the exception of the adduct S4N4.CuC1. The coordination of the AsF 5 group to one of the N atoms results in the opening of the SaN 4 cage structure to a puckered boat-shaped ring with the four S atoms in a plane.Introduction. During attempts to prepare cationic derivatives of 84N4, the reactions of S4N 4 with the compounds Ss(AsF6) 2 and Ses(AsF6) 2 in SO 2 have been observed to give the species S6N4(AsF6) 2 and its selenium analogue Se4S2N4(AsF6)2 (GiUespie, Kent & Sawyer, 1980). However, when the corresponding reactions of S4N 4 with both Te4(AsF6) 2 in SO 2 and Ses(AsF6) 2 in methylene chloride were attempted, they gave a deep-red crystalline product which has been characterized by X-ray crystallography as a 1:1 adduct of S4N 4 with the Lewis acid AsF 5.Crystals of S4N 4. ~sF 5 exist in a variety of forms depending on the reaction conditions. An approximately spherical crystal of this material was selected from the product of the reaction of Sea(AsF6) 2 with S4N 4 and sealed in a Lindemann capillary. Accurate unit-cell dimensions were obtained by least-squares refinement with 15 high-angle reflections measured on a Syntex P2~ diffractometer using Mo Ka radiation.(2 = 0.71069 A). Data were originally collected to a maximum 20 of 50 ° using a B-centred cell a' = 5.593, b'= 11.062, c'= 14.561 A withfl' = 92.33 ° and later transferred to the primitive cell above. Reflections were measured using 0-20 scans over a 20 scan range (Ka~ -0.9 °) to (Ka 2 + 0.9 °) and variable scan rates of 4-29.3 ° min -~ depending on the intensity of preliminary count. A total of 2260 data in the