= Solution
\b[$B$ is a 1:1 <nitrogen>-bound adduct, with $\mathrm{N\rightarrow AlBr_3}$ at one ring <nitrogen>.] Its <lone pair> is diverted from N-to-B pi donation into the new coordinate bond. That <nitrogen> becomes approximately tetrahedral and the ring can pucker. The two B–N bonds adjacent to it lose pi character and lengthen; the neighboring unbound <nitrogen> atoms can donate more strongly into those borons, shortening the next pair of B–N bonds. The remaining pair forms a third class. The second structural panel marks these three two-bond sets; it represents connectivity and bond classes, not a measured geometry.
In the slow-exchange structure, two boron atoms flank the bound <nitrogen> and are equivalent to one another, while the third boron is unique. This gives \b[two $^{11}$B environments in a $2:1$ ratio]. At room <temperature>, rapid exchange of the <Lewis acid> among the three equivalent <nitrogen> sites averages every boron over the same environments, giving one <boron-11 nuclear magnetic resonance> signal. Cooling slows exchange relative to the resonance-frequency difference and resolves the two environments.
This is <Chemical exchange in NMR>, not restoration of equal instantaneous B–N bonds. A spectrum alone cannot distinguish intramolecular migration from reversible dissociation and reassociation; concentration dependence and crossover experiments would test that distinction. Related N-bound <borazine> adducts have been characterized by diffraction and variable-<temperature> <NMR>, supporting the structural logic while not supplying the exact <temperature> dependence of this particular hexamethyl compound.<Aluminium and gallium trihalide adducts of 2,4,6-triorganylborazines and the synthesis of triorganylborazinium tetrabromoaluminates by B. Gemünd and others>[2008, pp. 137–142, N-bound adduct structures and variable-temperature NMR]{ref}
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