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Your textbook writes aluminium chloride as AlCl3. Measure the relative formula mass of its vapour, though, and it comes out as Al2Cl6, twice as heavy, according to the LibreTexts page on coordinate bonding/Chemical_Bonding/Fundamentals_of_Chemical_Bonding/Coordinate_(Dative_Covalent)_Bonding).
Why does aluminium chloride exist as a dimer, Al2Cl6?
Start by counting electrons. Aluminium has three valence electrons, and in AlCl3 it uses all three to make three Al–Cl bonds. That leaves only six electrons around aluminium, two short of an octet. AlCl3, like BF3, is electron deficient, as the same LibreTexts page puts it.
Each chlorine, though, still carries three lone pairs. So two AlCl3 units fix each other's shortage: one chlorine from each donates a lone pair to the other aluminium. The same page says the bonds between the halves are coordinate bonds and that forming them releases energy, so the dimer is more stable than two AlCl3 molecules.
After dimerising, each aluminium has four chlorines around it, which means four electron pairs: a full octet.
How do you draw the structure of Al2Cl6?
Use this five-step method, then count electrons.
Draw two Al atoms side by side.
Put one chlorine between them above and one below. These bridging chlorines bond to both Al atoms.
Give each Al two terminal chlorines on the outside.
In each bridge, draw one Al–Cl as a line and the other as an arrow from Cl to Al: the coordinate bond.
Four electron pairs around each Al means VSEPR predicts a roughly tetrahedral arrangement.
Now the check. Two Al and six Cl bring 2 × 3 + 6 × 7 = 48 valence electrons. Eight Al–Cl bonds use 16 of them. The other 32 make 16 lone pairs: three on each of the four terminal chlorines (12) and two on each of the two bridging chlorines (4). Nothing is left over.
Students often mix this up with diborane. In B2H6, hydrogen has no lone pair, so its bridges are three-centre, two-electron bonds; see our post on diborane's three-centre bonds. In Al2Cl6, chlorine brings its own lone pair, so every bridge bond is an ordinary two-electron bond.
When is it Al2Cl6 and when is it AlCl3?
It depends on temperature, so state conditions in an exam.
Solid at room temperature: an ionic lattice with a lot of covalent character, with each aluminium surrounded by six chlorines, according to the LibreTexts page on Period 3 chlorides/Descriptive_Chemistry/Elements_Organized_by_Period/Period_3_Elements/Chlorides_of_Period_3_Elements).
Around 180 °C: at atmospheric pressure the solid sublimes at about 180 °C (above about 2 atmospheres it melts at 192 °C instead), and the lattice converts to molecular Al2Cl6, with each aluminium surrounded by four chlorines (same LibreTexts page).
Melt and vapour: both contain Al2Cl6 dimers, according to Wikipedia's aluminium chloride article.
Higher temperatures: the dimers split into trigonal planar AlCl3 monomers, shaped like BF3 (same article).
Heat breaking a dimer apart is a pattern you will meet again, for example when NO2 dimerises to N2O4 at low temperature.
Why does the dimer break up in water or donor solvents?
The bridge exists only because aluminium wants one more electron pair. Offer it another donor that binds in its place, and the chlorine bridge is no longer needed.
In water, oxygen lone pairs take over completely. LibreTexts gives the reaction as AlCl3 + 6H2O → [Al(H2O)6]3+ + 3Cl−, and notes that a solution of around 1 mol dm−3 has a pH of about 2–3, while a drop of water on the solid gives off HCl gas (LibreTexts, Period 3 chlorides/Descriptive_Chemistry/Elements_Organized_by_Period/Period_3_Elements/Chlorides_of_Period_3_Elements)). The acidity is explained in our post on why aluminium salt solutions are acidic.
In ethers, an oxygen lone pair coordinates to aluminium instead of a bridging chlorine. A 1982 27Al NMR study found AlCl4− in concentrated diethyl ether solutions, and AlCl3·OR2 alongside it in dilute solutions at low temperature; in tetrahydrofuran the main species in equilibrium were AlCl3·2THF, AlCl4− and AlCl2(THF)2+ (Zeitschrift für Naturforschung B, 1982). A chloride ion does the same job: AlCl3 forms tetrachloroaluminate, [AlCl4]−, when chloride is present (Wikipedia).
How do you avoid losing marks on this question?
Write "electron deficient" and "six electrons around Al".
Name the bond: coordinate (dative), from a chlorine lone pair to aluminium.
Draw two bridging and four terminal chlorines, with two arrows.
Never call the bridges three-centre, two-electron bonds. That is diborane.
Give conditions: dimer near sublimation, monomer when hotter, neither in water.
If electron deficiency itself feels shaky, the Fermi Concept Library gives each chemistry concept its own lesson and its place in the map. To practise, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
Tonight, draw Al2Cl6 and the ether adduct from memory. If both count to an octet, you have it.
