BCl3 is in trivalent state and has 6 electrons in its valence shell and need two more electrons to complete the octet. BCl3 act as a lewis acid and NH3 can donate its electron. BCl3readily accepts the lone pair of an electron from NH3 and form BCl3.NH3.
BCl3 + NH3 → BCl3.NH3.
AlCl3 forms a stable molecule by making a Dimer. Aluminium has 6 electrons in the valence shell and need 2 more electrons to complete the octet whereas chlorine has 3 lone pair of electron. Aluminium takes up 1 lone pair and forms Dimer with chlorine.
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