Editing Lewis acids and bases (section)

===Complex Lewis acids===
Most compounds considered to be Lewis acids require an activation step prior to formation of the adduct with the Lewis base. Complex compounds such as [[Ethylaluminium sesquichloride|Et<sub>3</sub>Al<sub>2</sub>Cl<sub>3</sub>]] and [[Aluminium chloride|AlCl<sub>3</sub>]] are treated as trigonal planar Lewis acids but exist as aggregates and polymers that must be degraded by the Lewis base.<ref>Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. {{ISBN|0-7506-3365-4}}.{{page needed|date=September 2021}}</ref> A simpler case is the formation of adducts of borane. Monomeric BH<sub>3</sub> does not exist appreciably, so the adducts of borane are generated by degradation of diborane:
:B<sub>2</sub>H<sub>6</sub> + 2 H<sup>−</sup> → 2 {{chem2|BH4-}}
In this case, an intermediate {{chem2|B2H7-}} can be isolated.

Many metal complexes serve as Lewis acids, but usually only after dissociating a more weakly bound Lewis base, often water.
:[Mg(H<sub>2</sub>O)<sub>6</sub>]<sup>2+</sup> + 6 NH<sub>3</sub> → [Mg(NH<sub>3</sub>)<sub>6</sub>]<sup>2+</sup> + 6 H<sub>2</sub>O