Illustrated Glossary of Organic Chemistry

Lewis acid-base adduct (Lewis acid-base complex): A molecule formed by the bonding of a Lewis acid with a Lewis base, without simultaneous loss of a leaving group. (In other words the Lewis acid-Lewis base reaction cannot be a substitution reaction.) A Lewis acid-Lewis base adduct (or complex) is usually depicted with a raised dot between the Lewis acid and Lewis base components (Lewis acid^.Lewis base), or (less commonly) an arrow indicating the electron donation from the Lewis base to the Lewis acid (Lewis base→Lewis acid).

Reaction of borane (BH₃;  a Lewis acid) with THF (a Lewis base) occurs when an oxygen lone pair forms an oxygen-boron bond. The reaction product is a Lewis acid-Lewis base adduct, because all of the reactant atoms are part of the product; there is no leaving group. The product can be abbreviated as BH₃^.THF or BH₃←THF.

Reaction of iodomethane (CH₃I; a Lewis acid) with hydroxide ion (HO^-; a Lewis base) to form methanol is not an example of Lewis acid-Lewis base adduct formation. Instead, it is an S_N2 reaction because iodide ion (the leaving group) departs.