Experiments have shown * that the ruthenium red cation is [Ru,O,(NH,), J6+ in which there are two oxo-bridges and the average oxidation number of ruthenium is + 10/3. The cation is oxidised reversibly by a one-electron change to the more paramagnetic brown cation [Ru,O,(NH,), J7+. The relation to other polynuclear complexes is discussed. JOLY formulated ruthenium red as Ru,Cl4(OH),,7NH3,3H,O, and Morgan and Burstall,2 who prepared from it supposed mononuclear complexes of ruthenium, as [Ru11x(OH)Cl(NH,),]C1,H20. The latter structure was rejected by Gleu and Breuel3 because [RuIrl(OH)C1(NH,),] + is paramagnetic whereas ruthenium red is diamagnetic.It has been suggested that the highly coloured cation is polynuclear, but further evidence on its structure has not been advanced.Our interest in ruthenium red arose from a view that the diamagnetism and intense colour of many complexes of ruthenium are due to their being polynuclear. In particular, the occurrence in solution of an aquochloro-* and an aquonitrato-complex 5 3 6 as transient violet species with unusually high molar extinction coefficients (cRU -20,000 at A , , -550 mp) suggested a chromatic group similar to that in ruthenium red for which we find that cRu is 21,000 at A, , , 532 mv. It has now been shown that this cation, here called ruthenium red, has no complexed halogen groups, and behaves as a trinuclear complex in which, associated with three ruthenium atoms, there are 14 nitrogen atoms which can be liberated as ammonia. This suggests that the red cation has two oxo-bridges, as in (I), to satisfy hexa-co-ordination of the ruthenium atoms. This cation is oxidised, reversibly, [(NH3),R~-O-R~(NH,)4-O-Ru(NH3)5]6+ [( N H 3)5 Ru-O-Ru (N H3I4-O-Ru (N HJJ ' + (I) Red (11) Brown in acid solution to a brown cation (11). in the respective cations are 10/3 and 1113. red cation points to a linear configuration of the Ru-O-Ru-O-Ru group.

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The products formed by gas-phase reactions between NH,, SO,, and water vapour diluted with air have been identified by chemical analysis and i.r. spectroscopy as ( NH. , ), SO, and (NH, ), S, O, .The dissociation constants of these compounds have been determined for the temperature range 273-298 K by a transpiration method. The results for complete dissociation fit the following Arrhenius equations for the sulphite and pyrosulphite respectively :53.29 f 0.01) -(14,171 f 33)lT and log (K,/m-lo Ns) = (66.92 f 0.02) -(17,705 f 183)lT. The relevance of these reactions to air pollution problems is discussed.THE products formed by the exothermic reaction, under anhydrous conditions, between the gases NH, and SO, have been studied previously1 and found to vary in composition and colour according to the temperature and relative proportions of the reacting gases. The primary products, formed as white solids at (283 K, were established as compounds in which NH, and SO, combined in stoicheiometric ratios of either 1 : 1 to give amidosulphurous acid (NH,*SO,H), or 1 : 2 to give ammonium amidosulphite (NH,-SO2NH4). Other workers have found evidence, from i.r. spectra, for the formation in a closed system of the gaseous compound, thionyl imide (HNSO), and ammonium pyrosulphite.Measurements of the equilibrium vapour pressures above the compounds NH,*SO,H and NH,*SO,NH, at 203-263 K, have indicated that these reaction products could be of importance in the formation of the stratospheric aerosol layer. Ammonium sulphite has long been recognised as an intermediate in the direct synthesis of ammonium sulphate from the gaseous constituents in an oxidising atmosphere. Calculations on the variation of the equilibrium constant with temperature for the formation of ammonium sulphite showed that the constant increases rapidly with increase in temperature and that at 453 K, the reaction is about half complete.Our object was to identify and determine the stability of the products formed by reaction of gaseous NH, with SO, in humid air, and to relate the results to conditions which are applicable to polluted atmospheres, in which the primary oxidation product, ammonium sulphate, is formed 6 by a droplet-phase mechanism. EXPERIMENTAL Preparative Methods.-The solid compounds were prepared a t room temperature (293 & 1 K) by mixing two air streams (each ca. 40 ml s-l) containing variable proportions of either NH, and water vapour, or SO, and water vapour.The initial mixing of the gases was carried out in a 50-ml silica test-tube to give products A (with excess of NH,), or C (with excess of SO,). The emergent gas stream, after passing through a second silica test-tube, was mixed with a E.

Ammonia occurs in the troposphere chiefly as ammonia gas or as ammonium sulfate. The principal source of the ammonia, at least in the U.K., appears to be the urea in animal urine. The ammonia thus released is converted on a time scale of hours into ammonium sulfate, the conversion process being quite rapid in the dark, as well as in the light, and over a range of relative humidity (R. H.) values. The most significant mechanisms probably are reaction with sulfur dioxide and oxygen in mist or cloud droplets and reactions with sulfuric acid droplets from power‐station plumes. There is no positive evidence for appreciable reaction on solid particulates, but the possibility cannot at present be excluded. Photochemical reactions probably make only a small contribution. A direct gas‐phase reaction between ammonia and sulfur dioxide does not seem possible at the concentrations and temperatures concerned. It is important to elucidate the mechanisms so that ammonium sulfate levels in the troposphere and their effects, e.g., on visibility, can be predicted.