ON THE PENTAVALENT NITROGEN ATOM AND NITROGEN PENTACOORDINATION

Hydrazoic acid (HN3) is an example of a molecule whose bond lengths suggest that the central nitrogen atom is apparently pentavalent, as indicated in the classical valence bond structure (I). [GRAPHICS] However, unless the nitrogen atom expands its valence shell, the pi bonds of this structure are fractional electron-pair bonds. The increased-valence structure (II) with fractional electron-pair pi and pi bonds, and 1-electron pi and pi' bonds, also involves an apparent pentavalence. Some of the properties of these two VB structures are used to restate the nature of the origin of the apparent electronic pentavalence for nitrogen, namely appreciable contributions of Dewar-type structures such as (III) to the component Lewis structure resonance scheme. It is shown that although the valence of the central nitrogen atom of structure (II) is able to exceed a value of 4, it can never attain a value of 5. Increased-valence structures for the C2 isomer of N6 are also presented, and the bond lengths that are associated with the most stable of these structures are in accord with those calculated using ab initio techniques. The results of some ab initio VB calculations, with minimal basis sets, are reported for: (a) N2, to demonstrate the effect of variation in sigma bond atomic orbital hybridization on the lengths of the N''-N''' bonds of HN3 (as HN'-N''-N''') and the N-N bond of N2; (b) trigonal bipyramidal NH3F2 and PH3F2, to suggest that the unwillingness of nitrogen to form stable pentacoordinate compounds is associated with some reluctance by nitrogen to participate in the formation of axial 4-electron 3-centre sigma bonding units.