= Solution
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.
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