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A Lewis acid-base adduct in which the ammonia nitrogen lone pair donates into the empty boron-centered acceptor orbital of borane. A coordinate covalent bond links nitrogen and boron.

Hexamethylborazine is ${\mathrm{B}}_3{\mathrm{N}}_3\mathrm{M}{\mathrm{e}}_6$, with one methyl on every ring atom. Its nitrogen atoms can form Lewis acid-base adducts, while nucleophiles can attack its boron atoms.

A Lewis acid-base adduct forms when an electron-pair donor binds an acceptor through a coordinate covalent bond. A ring nitrogen can bind $\mathrm{GaC}{\mathrm{l}}_3$ without displacement of a chlorine substituent from phosphorus.

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 ${\sigma }^{\ast }$ 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.^[ref]

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.^[ref]