Give reasons for the following: 1. [SiF_6]^ 2- is known whereas [… - Vidyadip

Question

Give reasons for the following:

$[\text{SiF}_6]^{2-}$ is known whereas $[\text{SiCl}_6]^{2-}$ is not known.
Diamond is covalent, yet has high melting point.
Boric acid is considered as a weak acid.
Boron is unable to form $\text{BF}^{3-}_6$ ion.
$\text{BF}_{3}$ behaves as a Lewis acid.

✓

Answer

i. $\left[\mathrm{SiF}_6\right]^{2-}$ is known whereas $\left[\mathrm{SiCl}_6\right]^{2-}$ is not known since six large size atoms i.e. six chlorine atoms cannot be accommodated around silicon atoms but six small size atoms (F atoms) can be comfortably accomodated.
ii. Diamond is a covalent solid but has a high melting point due to its three dimensional network structure.
iii. Boric acid is not able to release $\mathrm{H}^{+}$ions on its own, hence is a weak acid. It first receives $\mathrm{OH}^{-}$ions from water molecule to complete its octet and inturn releases $\mathrm{H}^{+}$ions.
iv. Boron is unable to form $\mathrm{BF}_6^{3-}$ ion due to the non availability of d-orbitals.
v. Boron in $\mathrm{BF}_3$ has only six electrons and hence can accept a pair of electron, thereby can act as Lewis acid.

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