Scheme I1A second binary compound, tetrasulfur dinitride (S4N2, 4), was also isolated before the end of the last century (32). Its correct elemental composition, however, was not established until 1925 (33), and its molecular formula not confirmed until 1951 (34). Early estimates suggested the formula N2Sj (32), an observation that attests to the tendency of the molecule to disproportionate above its melting point of 22.5"C (hence, the high sulfur content). The six-membered ring structure with a half-chair conformation was finally confirmed in 1981 (35,36). Most early preparative methods involved a thermal decomposition, for example, of S4N, in the presence of sulfur and/or carbon disulfide (37,38). More recently the use of simple metathetical reactions, for example, between Ni(S2N2H)? and S2CI2 has been explored (39). A simple procedure involving the reaction of aqueous ammonia with sulfur monochloride has also been developed (40); although the yield is low (<2%), the method provides a convenient preparation of small quantities (<0.5 g) starting from commercially available materials (3%). The solid state polymerization of S4N2 to (SN), ( 5 ) has recently been reported (36); this process is presumably initiated by trace amounts of (radical) impurities since, when recrystallized from diethyl ether, solid S,N, is indefinitely stable below its melting point (3%).Disulfur dinitride (6) (Scheme 11) was first generated (inadvertently) by Burt in 1910 (41) during an attempt to remove sulfur impurities from SJNJ by passing a vaporous mixture of the two over finely divided silver. At the time he noted the formation of a blue film [later characterized as (SN), polymer]. This reaction has been studied extensively since then, and now 304 RICHARD T. OAKLEY
