In molecular boron trichloride, boron uses three approximately hybrids to form three equivalent B–Cl sigma bonds in a trigonal plane. Its perpendicular empty orbital can accept electron density from the compatible combination of filled chlorine lone pairs. The resulting pi donation is delocalized over the three B–Cl connections.
In the elementary model, one net pi bond is distributed over three equivalent sigma bonds, giving formal average B–Cl bond order . The corresponding resonance contributors each put a B=Cl connection in a different position, with formal negative boron and positive donating chlorine. They do not mean one experimentally distinct double bond and two single bonds. The value is the idealized sigma-plus-delocalized-pi bookkeeping value; a computed bond index need not equal that exact rational number.
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 fragment donates into an empty orbital of the other Lewis acid centre; reciprocal donation closes the four-membered 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).
Dimerization lowers enthalpy by letting the coordinatively unsaturated aluminium centres accept chlorine lone-pair donation. Effective intramolecular Cl-to-Al pi stabilization of the monomer is relatively weak, while aluminium's larger size accommodates four-coordinate geometry and two chloride bridges. At room temperature this bonding gain can outweigh the entropy penalty of associating two molecules.
In boron trichloride, the compact short B–Cl framework supports appreciable ligand-to-boron pi donation, stabilizing the planar monomer. Its smaller boron centre also makes the crowded four-coordinate bridged arrangement less favourable. Thus dimerization must be assessed from the balance of monomer stabilization, bridge-bond formation, structural strain and entropy; merely counting six electrons around each monomeric centre would incorrectly predict identical association for both. orbital overlap depends on radial extent, bond length and energy matching, not just whether two orbitals share the same principal quantum number.
Heating increases the monomer fraction through . Dissociation breaks the bridge interactions and gains translational entropy by making two gas molecules from one. Its enthalpy cost becomes easier to offset at higher temperature. The qualitative statement concerns the molecular gas-phase equilibrium at the stated conditions, rather than every solid or liquid phase of aluminium chloride.
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