The reaction of SeCl2 with tert-butylamine in various molar ratios in THF at -78 degreesC has been investigated by Se-77 NMR spectroscopy. In addition to the known Se-N heterocycles Se-6((NBu)-Bu-t)(2) (1) and Se-9((NBu)-Bu-t)(6) (2), the acyclic imidoselenium(II) dichlorides ClSe[(NBu)-Bu-t)Se](n)Cl (4, n = 1; 5, n = 2) and two new cyclic selenium imides [Se-3((NBu)-Bu-t)(2)](n) (3, n = 1 or 2) and Se-3((NBu)-Bu-t)(3) (6) have been isolated and identified. An X-ray analysis shows that 6 is a six-membered ring in a chair conformation with \d(Se-N)\ = 1.833 Angstrom. Crystal data: 6, trigonal, P3c1, a = 9.8660(3) Angstrom, c = 20.8427(7) Angstrom, V = 1757.0(1) Angstrom (3), Z = 6. The H-1, C-13, and Se-77 NMR data for 1-6 are reported, and some reassignments of earlier literature data for 1-3 (n = 1) are made. The decomposition of (BuN)-Bu-t=Se=(NBu)-Bu-t at 20 degreesC in toluene was monitored by Se-77 NMR. The major products are 6 and 3. The Se(IV)-N systems (BuNSe)-Bu-t(mu -(NBu)-Bu-t)(2)E (7, E = SO2; 8, E = SeO) were prepared by the reaction of a mixture of SeCl4 and excess (BuNH2)-Bu-t with SO2Cl2 or SeOCl2, respectively. Compound 8 is also generated by the cycloaddition reaction of (BuNSeNBu)-Bu-t-Bu-t with (BuNSeO)-Bu-t. Both 7 and 8 consist of slightly puckered four-membered rings. The mean terminal and bridging Se-N distances in 7 are 1.665(2) and 1.948(2) Angstrom, respectively. The corresponding values for 8 are 1.687(4) and 1.900(4) Angstrom, and d(Se=O) = 1.628(4) Angstrom. Crystal data: 7, monoclinic, P2(1)/c, a = 18.669(4) Angstrom, b = 12.329(2) Angstrom, c = 16.463(3) Angstrom, beta = 115.56(3)degrees, V = 3418.4(11) Angstrom (3), Z = 4; 8, triclinic, P (1) over bar, a = 6.372(1) Angstrom, b = 103.876(3) Angstrom, V = 841.3(3) Angstrom (3), Z = 2. 9.926(2) Angstrom, c = 14.034(3) Angstrom, alpha = 99.320(3) Angstrom, beta = 95.764(3) Angstrom, gamma = 103.876(3) Angstrom, V = 841.3(3) Angstrom (3), Z = 2/.