The reaction of (Me3SiN)(2)S with SeCl2 (2:1 ratio) in CH2Cl2 at -70 degreesC provides a route to the novel mixed selenium-sulfur-nitrogen compound (Me3SiNSN)(2)Se (1). Crystals of 1 are monoclinic and belong the space group P2(1)/c(1) with a = 7.236(l) Angstrom, b = 19.260(4) Angstrom, c = 11.436(2) Angstrom, beta = 92.05(3)degrees, V = 1592.7(5) Angstrom(3), Z = 4, and T = -155(2) degreesC. The NSNSeNSN chain in 1 consists of Se-N single bonds (1.844(3) Angstrom) and S=N double bonds (1.521(3)-l.548(3) Angstrom) with syn and anti geometry at the N=S=N units. The N-Se-N bond angle is 91.8(1)degrees. The El mass spectrum shows a molecular ion with good agreement between the observed and calculated isotopic distributions. The N-14 NMR spectrum exhibits two resonances at -65 and -77 ppm. Both C-13 and Se-77 NMR spectra show single resonances at 0.83 and 1433 ppm, respectively. The reaction of 1 with an equimolar amount of SeCl2 produces 1,5-Se2S2N4 (2) in a good yield, and that of (Me3SiNSN)(2)S with SCl2 affords S4N4 (3), but the reactions of (Me3SiNSN)(2)Se with SCl2 and (Me3SiNSN)(2)S with SeCl2 result in the formation of a mixture of 2 and 3. A likely reaction pathway involves the intermediate formation of E2N2 fragments (E = S, Se).