Boron nitrosyl was examined with DFT. MP2. CCSD(T). and CASSCF. Poor overlap of the atomic orbitals on boron with the molecular orbitals of NO causes an accidental near degeneracy of the highest occupied sigma-and pi-orbitals. A near degeneracy of three electronic states results. DFT locates all three electronic states but finds unstable solutions for the (3)Sigma(-) and (3)Pi linear isomers. CASSCF/6-311+G(3df) and MP2/6-311+G(2df) predict.,(3)Sigma(-) and (3)Pi isomers but do not locate the (3)A" bent structure. CCSD(T)/6-311+G(d). predicts the (3)A" bent isomer to be 1.7 and 4.2 kcal/mol lower than the (3)Sigma(-) and (3)Pi linear isomers. respectively. (C) 2002 Elsevier Science B.V. All rights reserved.