Question
$\mathrm{BCl}_3$ exists as monomer whereas $\mathrm{AlCl}_3$ is dimerised through halogen bridging. Give reason. Explain the structure of the dimer of $\mathrm{AlCl}_3$ also.
✓
Answer
$\mathrm{BCl}_3$ exists as monomer in spite of the fact that it is an electron deficient compound. It achieves stability by forming $p \pi$ - $\mathrm{p} \pi$ back bonding. Chlorine transfers two electrons to vacant $2 p$-orbital of boron. All the three bond lengths are same. Thus, the back
bonding giving double bond character is delocalized in the molecule. $\mathrm{AlCl}_3$ is also an electron deficient compound. Since, aluminium has larger size than boron, the back bonding is not possible in $\mathrm{AlCl}_3$. Aluminium metal complete its octet by forming coordinate bond with chlorine atom of the other molecule, $\mathrm{AlCl}_3$. Thus, coordinate bond forming bridges by chlorine atoms between two Al atoms make a dimer molecule.
bonding giving double bond character is delocalized in the molecule. $\mathrm{AlCl}_3$ is also an electron deficient compound. Since, aluminium has larger size than boron, the back bonding is not possible in $\mathrm{AlCl}_3$. Aluminium metal complete its octet by forming coordinate bond with chlorine atom of the other molecule, $\mathrm{AlCl}_3$. Thus, coordinate bond forming bridges by chlorine atoms between two Al atoms make a dimer molecule.
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