while aluminium can form the ion (AlF6)3-, boron is unable to form (BF6)3- ion
As the valence shell in B is L shell it can accommodate a maximum of 8 (2x22) electrons, whereas in case of (BF6)3- the no. of electrons around central atom is 12 which is not possible.
In case of Al the valence electron is in the M shell and can accommodate a maximum of 18 electrons (2x32) hence (AlF6)3- can exist.
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because of oxidation states