Explain the following :
(a) Boron has high melting and boiling points.
(b) The `p pi - p pi` back bonding occurs in the halides of boron and not in those of aluminimum.
( c) Boron and aluminium halides behave as Lewis acids.
(d) Aluminium forms `[AlF_(6)]^(3-)` ion, but boron does not form `[BF_(6)]^(3-)` ion.

Explanation

(a) Boron has gaint covalent polymeric structure both in solid and liquid states, thus it has high melting and boiling points.
(b) The formation of `p pi- p pi` back bonding involves donation of an electron pair from the filled orbital of the atom forming a `'sigma'` bond with the central atom. The tendency to show back bonding depends on the size of the atoms involved. The tendency decreases as the size of the central atom increases. Since boron has smaller size as compared to aluminium, back bonding is more feasible in boron halides as copared to aluminium.

( c) Both boron and aluminium in their halides possess six electrons in their valence shell. Because these are short of two electrons to complete their octet and thus behave as electron-deficient compouds. They can easily accepy a pair of electron frm a donar ion//molecule and thus behave as Lewis acid.
(d) Due to the absence of low-lying vacant d-orbitals in boron, it cannot expand its coordination number beyond four and thus boron does not form `[BF_(6)]^(3-)` ion. On the other hand, aluminium due to the presence of vacant d-orbitals in valence shell can expand its coordination number beyond `4 (up to 6)` and thus easily from `[AlF_(6)]^(3-)` ion.