Type 11 Lewis Acid/Base Complexation Chemistry
Generally Onium ion Lewis acids do not complex with Lobe-HOMO Lewis bases, because the Onium ion is liable to transfer a ligand to the Lobe-HOMO Lewis base.

NH4OH (ammonium hydroxide) is clearly a Type 11 Lewis acid/base complex. This species exists in an equilibrium state with NH3 + H2O.

For more information look in the Chemogenesis webbook sections on Lewis acids and Lewis bases and the Lewis acid/base interaction matrix.