Lewis adduct

Ammonia gives a pair of electrons to boron trifluoride, making a Lewis adduct. The acid part's shape and bond lengths change when the adduct forms.

In chemistry, specifically the Lewis acid-base theory, a Lewis adduct is the result of a Lewis base sharing a pair of electrons with a Lewis acid.

Lewis adducts have a special type of chemical bond called a dipolar bond: where most covalent bonds are thought of as having one electron from each atom, in a Lewis adduct, both of the electrons come from one side (the base). The atoms in the dipolar bond have opposite formal charges.