Question

How does electron deficient compound BF₃ achieve electronic saturation i.e., the fully occupied outer electron shells?

Answer 1

1. By multiple bonding, e.g., BF₃ in which a lone pair of electrons of each fluorine atom may be used in a B → F π-bond (back bonding) involving the vacant orbital on the boron atom. 

2. By formation of complexes in which electrons are received from a donor molecule, 

e.g., F₃B → NH₃. 

Boron compounds, thus, behave as Lewis acids.