Q. Which of the following reactions does not take place?
$\ce{BF3 + F^{-} ->BF4^{-}}$$\quad\quad\quad\quad\quad\quad\quad\quad\ldots\left(I\right)$
$\ce{BF3 + 3F^{-} ->BF6^{3-}}$$\quad\quad\quad\quad\quad\quad\quad\ldots\left(II\right)$
$\ce{AlF3 + 3F^{-} ->AlF6^{3-}}$$\quad\quad\quad\quad\quad\quad\ldots\left(III\right)$

Solution:

BF3 forms complex halides of the type [BF-4] 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, BF3-6 ion (sp3d2 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.

You must select option to get answer and solution