Valence-bond calculations on N2 and isoelectronic species

Valence-bond calculations are reported for the isoelectronic series of molecules and ions: N₂, CO, BF, NO⁺ and CN^?. The most important structures are N?N, C?O, B_π? F, N⁺?O and C?N. Hybridization of the 2s and 2p orbitals is important. Only two or three structures are required to obtain an energy lower than that obtained with the molecular orbital approximation. Structures in which the electronegative element loses a σ-orbital or gains a π-orbital are favored. π-bonds tend to be favored over σ-bonds. The bond in NO⁺ resembles that in CO, whereas that in CN^? resembles the bonding in N₂.