= Solution
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 $\mathrm{AlCl_3}$ fragment donates into an empty orbital of the other <Lewis acid> centre; reciprocal donation closes the four-membered $\mathrm{Al_2Cl_2}$ ring.
The localized <coordinate covalent bond> model has \b[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).
Back to article page
