BF forms complex halides of the type [BF] in which B atom has C.N. 4, it cannot extend its C.N. beyond 4 due to the nonavailability of d-orbitals in its configuration. Hence, BF ion (spd hybridisation) is not formed. On the other hand, Al can extend its C.N. beyond 4 due to the availability of d-orbitals in its configuration.