Topological analysis of chemical bonding in cyclophosphazenes

Chemical bonding in the cyclophosphazenes is studied from the point of view of the quantum theory of Atoms in Molecules (AIM). To that end, HF/6-31G** ab initio calculations are done on a collection of (NPX2)(3) derivatives f or a wide set of -X substituents, and its electron density, rho((r) over ri ght arrow, and pair density, rho ((2))((r) over right arrow (1)(r) over rig ht arrow (2)), are obtained and analyzed. The (NP)3 ring geometry and bondi ng properties are basically maintained along the cyclotriphosphazenes. The PN distance and the bond critical point properties (electron density, Lapla cian, etc.) lie between those of XNPX3, formally a double NP bond, and thos e of X2NPX4, formally a single NP bond. being much closer to the former tha n to the latter. The Laplacian of the electron density shows the PN bond to be highly polar, with a clear tendency of the P atoms to lose almost all o f their five valence electrons, and a significant concentration of charge a long the PN line, even though within the N basin. The charge on the ring N basins, P(N), remains almost invariant, -2.3 e, in all cyclotriphosphazenes , whereas the charge of the ring P basin, (2(P), varies from +2.9 to +4.0 e , depending on the electronegativity of the -X group. There is an inverse c orrelation between e(P) and the PN distance, the more electronegative -X gr oups shrinking the (NP)(3) ring more, even though only slightly. The partit ion of the pair densities indicates that some 0.63 electron pairs are share d between each P and its two N neighbors in the ring, this value being typi cal of a polar but largely ionic bonding situation. The three N atoms in th e ring share 0.20 electron pairs per N-N group, a small but significant amo unt, even though no bond path line occurs linking them. The three-dimension al contour surfaces of del (2)rho clearly depict the molecular regions havi ng a Lewis basic or acidic character. Ring N atoms behave as weak Lewis bas es, whereas ring P atoms are preferred sites for a nucleophilic attack tend ing to remove, perhaps ionically, a -X group. These topological properties do explain the chemistry of cyclophosphazenes and agree well with the avail able experimental densities. The AIM analysis supports the main conclusions from the traditional Dewar's model of phosphazenes.