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

Explanation

By the following two ways
(i) Multiple bonding of `ppi-ppi` back bonding , e.g., `BF_(3)` in which a lone pair of electrons present in `2p-`orbital of one of the fluorine atoms may be transferred to the vacant p-orbital one the boron atom.
(ii) Formation of complexes in which electronsare received from a donor molecule e.g., `F_(3)Brarr NH_(3)`.
Boron compounds, thus, behaves as Lewis acids.