A donor atom stabilized borylnitrene, 2-nitreno- 1,3,2-benzodioxaborole 4c, is characterized by matrix isolation IR, UV, and ESR spectroscopy as well as multiconfiguration SCF and CI computations. UV irradiation ( lambda = 254 nm) of the corresponding azide 6c, isolated in solid argon at 10 K, produces 4c in high yield. The oxygen donor atoms in 4c result in a triplet ground state (vertical bar D/ hc vertical bar 1.492 cm(-1), vertical bar E/ hc vertical bar = 0.004 cm(-1)) for the borylnitrene. The lowest energy singlet state ((1)A(1)) is 33 kcal mol(-1) higher in energy and closely related to the ground state of vinylidene. Under the conditions of matrix isolation, triplet 4c is photochemically and thermally stable toward rearrangement to the corresponding cyclic iminoborane. Photochemical irradiation ( lambda > 550 nm) of 4c rather causes an efficient reaction with molecular nitrogen, lying in matrix sites nearby, to give 6c. Similarly, photochemical, but not thermal, trapping of 4c with CO is possible and results in the corresponding isocycanate 9c. Thermal reaction of 4c with O-2 in doped argon matrixes at 35 K could be observed by IR spectroscopy to result in borylnitroso-O-oxide 17c as shown by O-18(2) labeling experiments and DFT computations. The diradical 17c is very photolabile and quickly rearranges to the nitritoborane 16c upon irradiation.