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(p pi-p pi) back bonding occurs in the ...

`(p pi-p pi)` back bonding occurs in the halides of boron but not in those of aluminium. Explain.

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The tendency of the central atom (A) in `AX_(3)` molecules `(A = B` or `Al, X = ` halogens) to form `(p pi-p pi)` back bonding depends on the size of central atom (A). Smaller is the size of the central atom, greater is the tendency to form `(p pi-p pi)` back bonding. Since boron (B) atom is smaller in size than aluminium `(Al)` atom, `BX_(3)` molecules have `(p pi-p pi)` back bonding while `AlX_(#)` molecules do not have this type of bonding.
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Boron, the first member of group 13 (p-block) is a typical non-metal. H exists in two allotropic forms. The compounds of boron are mainly electron deficient as well as Lewis acids. For example, all the trihalides of boron (BX_(3)) are Lewis acids. The expected order of acidic strength based on the electro negativity of halogen atoms is : BF_(3)gtBC l_(3)gtBBr_(3)gtBl_(3) . But the actual order is the reverse. This is explained with the help of the concept of back bonding (p pi- p pi) . Among the hydrides of boron. diborane has a bridged structure. In the reaction: BF_(3)+3LiBH_(4)rarr3liF+X is