= Solution
The six P–N bonds are not well described as three permanently localized single bonds alternating with three ordinary double bonds. The strongly polar P–N framework has contributions from <ionic bonding> and <nitrogen> <lone pair> donation into neighboring phosphorus-centered acceptor combinations, including P–Cl $\sigma^*$ orbitals. This <negative hyperconjugation> supplies additional P–N bonding and equivalent environments around the ring. Approximate planarity is compatible with these interactions, but equal lengths and planarity alone do not establish benzene-like <aromaticity>. Large phosphorus $3d$ participation is not required.<Molecular Structures and Conformations of Polyphosphazenes—A Study Based on Density Functional Calculations of Oligomers by Huai Sun>[1997, abstract and bonding analysis, ionic sigma bonding and nitrogen-to-P–X sigma-star donation]{ref}
\b[Gallium binds to a ring <nitrogen>, and the two P–N bonds flanking that <nitrogen> lengthen.] Formation of the N$\to$GaCl$_3$ <coordinate covalent bond> changes the available <nitrogen> <electron> density and local geometry, weakening its contribution to the adjacent P–N interactions. The other four P–N bonds retain greater multiple-bond character and are correspondingly shorter. Loss of the threefold equivalence and altered local geometry permit the ring to pucker. This is a <Lewis acid-base adduct>, not replacement of a P–Cl bond by GaCl$_3$. Single-crystal structures and variable-temperature multinuclear <nuclear magnetic resonance> establish this class of nitrogen-bound group-13 adducts.<Group 13 Lewis acid adducts of [PCl2N]3 by Zin-Min Tun and others>[2011, pp. 8937–8945, adduct structures and variable-temperature NMR]{ref}
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