Question

Aluminium (III) Chloride forms a dimer because

• A. higher coordination number can be achieved by aluminium
• B. aluminium has high ionisation energy
• C. aluminium belongs to III group
• D. it cannot form a trimer

Answer 1

The correct option is A higher coordination number can be achieved by aluminium

$AlC{l}_3$ can form dimer and exists as $A{l}_{2}C{l}_6$, aluminium has vacant $d$-orbitals which can accommodate electron from chlorine atom. $AlC{l}_3$ is an electron deficient compound in $Al$ (octet incomplete) thus behaves as Lewis acid and $Al$ completes it by taking electron pair from $Cl$-atom as shown in figure.