${\mathrm{BF}}_3$ easily reacts with Lewis base such as ${\mathrm{NH}}_3$ to complete octet around boron. This can be expressed as

The monomeric trihalides of elements of group 13, being electron deficient, are strong Iewis acids. Boron trifluoride easily reacts with Lewis bases such as ${\mathrm{NH}}_3$ to complete its octet around boron.

${\mathrm{F}}_3 \mathrm{B}+:{\mathrm{NH}}_3⟶{\mathrm{F}}_3 \mathrm{B}\leftarrow {\mathrm{NH}}_3$