The thermal decomposition of Se(NAd)(2) (Ad = 1-adamantyl) in THF was monitored by (77) Se NMR and shown to give the novel cyclic selenium imide Se-3(NAd)2 as one of the products. An X-ray structural determination showed that Se3(NAd)2 is a puckered five-membered ring with d(Se-Se) = 2.404(l) A and d(Se-N)j = 1.873(4) A. On the basis of 77 Se NMR data, other decomposition products include the six-membered ring Se3(NAd)3, and the four-membered rings AdNSe(mu-NAd)(2)SeO and OSe(mu-NAd)(2)SeO. The energies for the cycloclimerization of E(NR)(2) and RNEO (E = S, Se; R = H, Me, Bu-T, SiMe3 and the cycloaddition reactions of RNSeO with E(NR)(2), RNSO2 with Se(NR)2, and S(NR)2 with Se(NR)2 have been calculated at MP2, CCSD, and CCSD(T) levels of theory using the cc-pVDZ basis sets and B3PW91/6-31G* optimized geometries. Sulfur(IV) and selenium(]V) diimide monomers are predicted to be stable, the sole exception being Se(NSiMe3)(2) that shows a tendency toward cycloclimerization. The cycloclimerization energy for RNSeO and the cycloaddition reaction energies of RNSeO with Se(NR)2 as well as that of RNSO2 with Se(NR)2 are negative, consistent with the observed formation of OSe(mu-(NBU)-B-t)(2)SeO, OSe(mu-(NBU)-B-t)(2)SeO, and O2S(mu-(NBU)-B-1)(2)(SeNBu)-Bu-1, respectively. Cycloaddition is unlikely when one of the reactants is a sulfur(IV) diimide.