Type 3 complexes form when Proton Lewis acids complex with Lobe-HOMO (lone-pair) Lewis bases. The resulting complexes are Brønsted acids. In fact the vast majority of organic and inorganic Brønsted acids are Type 3 complexes. The pKa values of Type 3 complex species may vary from less than -12 (HI) to greater than 70 (tertiary alkanes), ie by more than 80 orders of magnitude! For more information look in the Chemogenesis
webbook sections on Lewis
acids and Lewis bases and the Lewis
acid/base interaction matrix.