Valence Bond Structures for N2O and HCNO with Apparently Pentavalent Nitrogen Atoms

For N2O, the results of STO-6G calculations indicate that a valence-bond structure I with a π electron configuration R[πx(NO)]1[πx(O)]1[πy(NO)]1[πy(O)]1, in which R = [πx(NN)]2[πy(NN)]2, generates a lower energy than does resonance between two valence-bond structures, II and III, with π electron configurations R[πx(NO)]2[πy(O)]2 and R[πy(NO)]2[πx(O)]2, respectively. In each of these three structures, the central nitrogen atom is apparently pentavalent. A similar conclusion is obtained from the results of the corresponding calculations for isoelectronic HCNO, with R = [πx(CN)]2[πy(CN)]2. Using HCNO + HCCH → isoxazole as the example, valence-bond representations for 1,3-dipolar cycloaddition reactions are compared using the above types of valence-bond structures. For N2O, the bond orders that are implied by structure I, but not those by structures II and III, are shown to be in qualitative accord with the observed bond lengths. The energy for structure I is also calculated to lie below that for a R[πx(NO)]2...