Spin-coupled description of fluorocyclophosphazenes (NPF2)3, (NPF2)4, (NPF2)5

Modern valence bond theory, in its spin-coupled form, is used to investigate the bonding in fluorocyclophosphazenes. We find that the spin-coupled orbitals for the ‘out-of-plane’ π system split into two sets. One of these consists of slightly deformed pπ functions that are fairly tightly localised on N atoms. The other set consists of functions that are also associated with individual N atoms but which also show significant delocalisation onto the neighbouring P atoms. The pairs associated with a given N atom are predominantly singlet coupled. It is found that the d basis functions used here for phosphorus play only a minor role, as polarisation functions. The same is true for the d functions on nitrogen. The spin-coupled description of the bonding, which is intermediate between the simplistic zwitterionic and covalent models, may be envisaged as a sequence of highly polarised Nδ-–Pδ+ bonds.