Why `[BF_(6)]^(2-)` ion does not exist?

Explanation

Valence shell electronic configuration of boron is `2s^(2)2p^(1)` i.e. boron can expand its coordination number to `4` only. Due to the absence of low-lying `d`-orbitals in its valence shell, boron cannot expand of low-lying `d`-orbitals in its valence shell, boron cannot expand its coordination number beyond `4`, hence,`[BF_(6)]^(2-)` does not exist.