Answer:
Both \[BC{{l}_{3}}\] and \[AlC{{l}_{3}}\]
are electron deficient molecules having six electrons in the valence shell of
their respective central atoms. To complete their octets, the central atom in
each case can accept a pair of electrons from the chlorine atom of another
molecule forming dimeric structures. However, because of small size of B, it
cannot accommodate four big sized Cl atoms around it. Therefore, \[BC{{l}_{3}}\]
prefers to exist as a monomeric planar molecule in which B atom is \[s{{p}^{2}}\]-hybridized
(Fig. 11.4, page 11/13).
On the other hand, Al because of its
bigger size can easily accommodate four Cl atoms around it. As a result, \[AlC{{l}_{3}}\]
exists as a dimer. In this dimer, since the covalency of Al has increased to 4.
Therefore, Al is\[s{{p}^{3}}\]-hybridized and the four Cl atoms are held
tetrahedrally around it. For structure, refer to Fig. 11.2, page 11/10.
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