(I) Size of boron atom is very small, so it cannot accomodate 4 large halide atoms and thus, does not form dimer. Also, the electron deficiency of boron is removed by pπ−pπ back-bonding. Hence, the boron trihalides do not form dimers. (II) In boron family, as we move down the group, two oxidation states are shown by them, i.e. +1 and +3 . However, boron shows only +3 oxidation state because of the absence of inert pair effect. Boron does not show +1 oxidation state. (III) Boron shows a maximum covalency of four because of absent of d-orbitals. (IV) Boron does not form BF66− ion because of absence of empty d-orbitals. Hence, only statements III and IV are correct.
