Bonding and aromaticity of cyclic phosphazenes viewed as interaction of Dnh fragments

The qualitative molecular orbital approach based on orbital interactions was used to explore the nature of bonding in cyclic fluorophosphazenes (F2PN)n, where n is 2-6. Besides the classical skeleton of σ-bonds, only two, one radial and one axial, 2n-center two-electron π-bonds significantly participate in the extra stabilization of the (PN)n ring. The π-radial interaction is more effective and comparable by size with the σ ones. Additional slight stabilization of a (PN)n ring is achieved by nonbonding π-radial and π-axial molecular orbitals (MOs) which are mainly localized on nitrogen atoms. The orbital interactions have a hyperconjugation character. The bonding energy decomposition analysis showed that the cyclic interactions are about half covalent and half electrostatic. The covalent bonding is dominated by radial interactions. The aromaticity concept is not appropriate for description of bonding in cyclophosphazenes. The contribution of phosphorus d atomic orbitals to the concept of chemical bonding in phosphazenes is negligible, but the inclusion of d phosphorus functions in a basis set is appropriate for a correct quantitative description of electronic and geometric structure. Extended Hückel (EHT), ab initio and density functional (DFT) calculations provide the same qualitative picture of the bonding. The very flat B3LYP/6-311+G(3df)//B3LYP/6-31G(3df) potential energy surface (PES) with the low energy barrier (∼1.6 kJ mol−1) indicates the very fast and easy conformational motion of (F2PN)4. The global minimum on this PES is the S4 conformation.