A valence bond study of the origin of the planarity of the C₂N₂O₂ units of cis and trans C-nitroso dimers

The results of some ab initio valence bond studies for cis, trans and perpendicular N-N dimers of HNO are reported, and used to provide a valence bond rationalization for the origin of the observed planarity, or near-planarity, of the C₂N₂O₂ units of cis and trans C-nitroso dimers. It is shown that as a consequence of some delocalization of the oxygen p electrons of the Lewis structures A and B: {A figure is presented} the planarity is associated with Pauling three-electron bond (N¨N° ? N°N¨) covalent-ionic resonance in the planar isomers, for example C?D + mirror-image structures for the cis isomer: {A figure is presented} as well as with the presence of an N-N electron-pair p bond in planar A and B. The relative importance of these two types of contributions depends on the extent of oxygen p electron delocalization. © 1993.