Question

$BC{l}_3$ exists as a monomer whereas $AlC{l}_3$ is dimerised through halogen bridging. Give a reason. Explain the structure of the dimer of $AlC{l}_3$ also.

Answer 1

$BC{l}_3\text{exists as a monomer}$

$BC{l}_3$ is electron deficient species and has $6$ electrons in the valence shell of central atom $B$.

It does not form dimer to complete its octet due to small size of boron because of which bridging $Cl$ atoms comes very closer in dimer and thus dimer becomes unstable. So, $BC{l}_3$ tries to exist as monomer.

$AlC{l}_3\text{dimer and structure}$

$AlC{l}_3$ exists as a dimer as the $Al$ atom in the molecule is electron deficient due to presence of only $6$ electrons in its' valence shell.


So, in order to gain stability and complete its octet, two $AlC{l}_3$ molecules dimerise through formation of coordinate bonds between lone pair of electron on $Cl$ atom in one $AlC{l}_3$ molecule and the empty $p$ orbital on $Al$ in another $AlC{l}_3$ molecule. thereby forming a dimer which consists of three-center-two-electron bonds at bridging position.

The bridging $Al$-$Cl$ bond is longer than the other $2$ $Al$ -$Cl$ bonds in the molecule. Both the $Al$ atoms in the dimer are tetrahedral shaped due to $s{p}^3$ hybridization of $Al$ atoms.