Answer:
Anhydrous HF
is a covalent compound and is strongly hydrogen bonded. It does not ionise and
therefore, ions do not react with \[Al{{F}_{3}}\] to give a \[{{[Al{{F}_{6}}]}^{3-}}\]complex
ion. However, when\[NaF\], an ionic compound, is added, \[{{F}^{-}}\] ions are
furnished and a complex is formed which is soluble.
\[Al{{F}_{3}}+3NaF\to \underset{\text{Soluble}\,\text{complex}}{\mathop{N{{a}_{3}}[Al{{F}_{6}}]}}\,\]
Boron, on account of small size and high electro negativity,
has much higher tendency to form complexes than Al, therefore, when \[B{{F}_{3}}\]
is added to the aluminium complex, \[Al{{F}_{3}}\] gets precipitated.
\[N{{a}_{3}}Al{{F}_{6}}+3B{{F}_{3}}\to
3Na[B{{F}_{4}}]+Al{{F}_{3}}\]
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