Revisiting the electronic structure of phosphazenes.

Natural bond orbital (NBO) and topological electron density analyses have been used to investigate the electronic structure of phosphazenes [N3P3R6] (R = H, F, Cl, Br, CH3, CF3, N(C2H4); 2R = O2C6H4), [N4P4Cl8], and H[NPCl2]4H. Using the former, the two most likely phosphazene bonding alternatives, negative hyperconjugation and ionic bonding have been critically evaluated. Ionic bonding, as suggested by topological analysis, was found to be the dominant bonding feature, although contributions from negative hyperconjugation are necessary for a more complete bonding description. Substituent effects on the P-N bond have been assessed and cases of bond length alternation have been rationalized using this combined bonding model, which supersedes previous models involving d-orbital participation, leading to an explanation for the observed bond length alternation found in some linear polyphosphazenes. In addition, common aromaticity indicators, nucleus independent chemical shifts (NICS) and para-delocalization indices (PDI), have been determined for the cyclophosphazenes.