Crystalline thermally stable (SeNSeNSe),(AsF,), containing the 'electron-rich aromatic' 6 7 t SeNSeNSe,' ( n = I) and 77t radical cation SeNSeNSe'+ ( n = 2) were prepared in high yields from reactions of Se,N, with stoichiometric quantities of Se,(AsF,), ( n = 1 ) or AsF, (n = 1 or 2) in liquid SO, and their X-ray crystal structures determined. The structure of SeNSeNSe(AsF,), consists of discrete planar SeNSeNSe2+ cations and AsF,-anions, and that of (SeNSeNSe),(AsF,), consists of two identical, but crystallographically different, discrete (SeNSeNSe"), cations and AsF,-anions. The centrosymmetric (SeNSeNSe'+),dimercontainstwo planar SeNSeNSe'+ radical cationsweaklyjoined bytwo long Se Se bonds [2 x 3.123(3), 2 x 3.149(3) A]. There are significant cation-anion interactions in both salts. The Se-Se [2.334(3) A] and Se-N bond lengths [average: 1.74(3) (side), 1.69(3) A (top)] in SeNSeNSe,' are shorter than their corresponding distances in the 77t SeNSeNSe'+ [average: Se-Se 2.398(3); Se-N 1.76(2) (side), 1.69(2) (top) A] consistent with removal of the unpaired electron from the x* singly occupied molecular orbital of the monocation. Surprisingly only one peak, rather than the expected two, was observed in the 77Se N M R spectrum of SeNSeNSe(AsF,), consistent with fluxional behaviour in solution. The 77Se chemical shift [-70 "C, G(Me,Se) = 2434, v f = 10 Hz] is the highest so far recorded and is consistent with the dipositive charge and 'electron-rich' 67r aromatic character. The "Se and 14N N M R [room temperature (r.t.), G(MeN0,) = -67.6, v-) =,200 Hz] and the Raman spectrum in liquid AsF, at 10 "C are all consistent with retention of the SeNSeNSe2+ ring structure in solution. The ESR t Supplemcwturj, data available (No. SUP 56904, 5 pp.): IR and FT-Raman spectra and mass spectral data for Se,N,. See Instructions for Authors,