Aluminium (III) chloride forms a dimer because aluminium

1. belongs to boron family.
2. has a very high ionisation enthalpy.
3. can have higher coordination number.
4. is next to magnesium.

Solution

$\mathrm{AlCl}_3$ can form dimer and exists as $\mathrm{Al}_2 \mathrm{Cl}_6$ because aluminium has vacant $d$-orbital which can accomodate electron from chlorine atom. $\mathrm{AlCl}_3$ is an electron deficient compound that completes its octet by taking electron pair from $\mathrm{Cl}$ atom.
So, Al can have higher coordination number.

Asked in: AP EAMCET 2021 (24 Aug Shift 2)

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