Question

AlCl3 makes dimers but BCl3 does not ?Why?

Answer 1

BCl3 and AlCl3 both are the example of electron deficient compounds, as there are only six electrons in the valency shell of central atom after the formation of the molecule, thus, both the compounds have strong tendency to gain (two) electrons, so that their octet is completed. In both the cases central atom undergoes sp2 hybridisation and both acquire trigonal planar geometry. The sp2 hybridisation involves one s and two p orbitals, and one p orbital remains unhybridised, this unhybridised p orbital lies perpendicular to the (trigonal) plane of the molecule which being empty, may recieve two electrons from a suitable donor so as to complete it’s octet. Now, remember, Chlorine atoms attached to the central atom possess lone pairs (three).

The electron deficiency (octet) of Boron in BCl3 is compensated (completed) by formation of co-ordinate bond between the lone pair of peripheral Chlorine atom and the empty unhybridised p orbital of Boron atom, forming pπ–pπ bonding. Thus, the electron deficiency of BCl3 is compensated by formation of pπ–pπ bonds within the molecule so no dimer forms.

However, the electron deficiency of Aluminium atom in AlCl3 is compensated (completed) by formation of co-ordinate bond between lone pair of Chlorine atom of another AlCl3 molecule and the empty unhybridised p orbital of Aluminium atom, thus forming dimer.

So, it is interesting to know that both BCl3 & AlCl3 are

halides of group 13 elements

Electron deficient

sp2 hybridised

Have empty one p orbital perpendicular to the plane of the molecule. But, former doesn't forms dimer and later forms a dimer, because both the molecules compensate their electron deficiency by different ways. Now, then the question arises is; why both these species are completing it's octet by different ways? This is because, Boron atom is so small that it cannot accommodate four bigger sized Chlorine atoms, however, Aluminium being bigger atom can do that efficiently and thus compensates it's electron deficiency by forming dimer. Hoping that this'll help you, Thank you!