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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.

A coordinate covalent bond is described as forming by donation of a lone pair from one partner into an acceptor orbital of the other. This formation description does not imply that the final electrons remain distinguishable by origin.

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.

A Lewis base donates an electron pair to a Lewis acid, forming a coordinate covalent bond in an adduct.

The nitrogen-centered highest occupied molecular orbital of ammonia supplies a lone pair; planar borane has an empty out-of-plane $p$-derived lowest unoccupied molecular orbital. Approach along this acceptor axis gives constructive orbital overlap and donation of the nitrogen pair into the boron-centered acceptor. The resulting N-to-B coordinate covalent bond forms the ammonia borane adduct.

The occupied molecular orbital formed by this interaction is bonding, while the corresponding antibonding combination remains empty. Borane pyramidalizes as coordination increases from three to four. Ammonia can use its existing outward-pointing lone pair; its inversion to the planar transition geometry is not a prerequisite. The Lewis base and Lewis acid descriptions summarize the donor and acceptor roles, while the frontier molecular orbitals explain their directional interaction.

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]

The dimer has four terminal chlorides and two bridging chloride ligands. Each aluminium is approximately tetrahedrally surrounded by two terminal and two bridging chlorines. A chlorine lone pair from one $\mathrm{AlC}{\mathrm{l}}_3$ fragment donates into an empty orbital of the other Lewis acid centre; reciprocal donation closes the four-membered $\mathrm{A}{\mathrm{l}}_2\mathrm{C}{\mathrm{l}}_2$ ring.

The localized coordinate covalent bond model has single Al–Cl bonds, formal bond order one, for both terminal and bridging connections. Donation describes the origin of a pair, not half an electron-pair bond. A bridge chlorine has two bonded pairs and two remaining lone pairs in this all-single-bond Lewis accounting; the original terminal chlorines have three lone pairs. Bridging contacts are longer and weaker than terminal ones, so equal formal order does not imply equal strength or equal numerical quantum-chemical bond indices. In particular, these chloride bridges should not be assigned the electron-deficient methyl-bridge half-bonds of (b).