Solve 30th one and give proper explanation
Q.30. Boron cannot form which one of the following anions?
(a) $B{F}_6^{-}$
(b) $B{H}_4^{-}$
(c) ${\left[B{\left(OH\right)}_4\right]}^{-}$
(d) $B{O}_2^{-}$
[Hint: Due to non-availability of d-orbitals, boron is unable to expand its octet. The maximum covalency of boron is 4.]

Dear student,
Boron has only three electrons in its outermost shell. It can share maximum three electrons hence can contain maximum six electrons in its valence state. 
Its covalency cannot extend the value four due to absence of vacant "d" orbital since 2d is not possible. Hence BF₆(-) is not possible. 

Hope it helps
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