How does electron deficient compound `BF_(3)` achieve electron saturation, i.e. fully occupied outer electronic shells?

Explanation

`BF_(3)` achieves electronic configuration by the following ways:
a.`ppi-ppi` back bonding, i.e. by donation of electron pair from filled `2p`-orbital of `F` to vacant `2p`-orbital of `B`.
b. Formation of complexes, i.e. by accepting a pair of electron from donor ion/molecule in vacant `2p`-orbital of `B`, e.g. `F_(3)Blarr:NH_(3)`. Boron compounds thus behaves as a Lewis acids.