When NH3 reacts with BF3, BF3-NH3 adduct is formed as shown in the image attached.
The reaction between NH3 and BF3 is an acid- base reaction. According to Lewis definition, an acid is any specie that accepts a pair of electrons while a base is any specie that donates a pair of electrons.
In this acid - base reaction, BF3 is the Lewis acid because it has an open sextet while NH3 is the Lewis base because it has a lone pair of electrons. The reaction of the both species produces a complex adduct as shown in the image attached.
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