acid-base adduct

When a Lewis acid and a Lewis base meet and stick, the single product they form is called an adduct (from the Latin for 'drawn toward'). It is the simplest possible result of donor-acceptor chemistry: two pieces, one bond, one new molecule, nothing else made or lost. The classic textbook adduct is F3B-NH3, formed cleanly from BF3 and NH3.

What holds an adduct together is a coordinate covalent bond, also called a dative bond: an ordinary covalent bond in every physical sense, except that both shared electrons originated from the base. We often draw it as an arrow pointing from donor to acceptor (H3N -> BF3) to record where the electrons came from, but once formed, the B-N bond is just a bond. Forming the adduct visibly changes the geometry: free BF3 is flat and trigonal-planar (boron sp2 hybridized), but in the adduct boron becomes sp3 and the molecule pyramidalizes, because boron now has four bonds and a full octet. The strength of an adduct (its dissociation energy) measures how well that particular acid and base match — and matching is exactly what the hard-soft principle predicts.

Adducts are everywhere in inorganic chemistry, just under different names. A coordination complex like [Co(NH3)6]3+ is simply a metal-ion Lewis acid that has bonded six ammonia Lewis bases — six adduct bonds at once. Solvent molecules coordinating to a metal, fluoride adding to BF3 to make BF4-, ethers stabilizing organolithium reagents, the famous 'borane-THF' reagent (H3B-O(C4H8)) used in organic synthesis — all are acid-base adducts. Recognizing a reaction as adduct formation immediately tells you a bond formed by lone-pair donation, and lets you predict geometry and reactivity changes at the acceptor atom.