Type 1 complexes form when Proton Lewis acids complex with s-HOMO Lewis bases (hydride ions, hydrogen or deuterium or tritium isotopic homologs). There are therefore of two general types of Type 1 Lewis acid/base complex: H2 and [H3]+. Both types of complex possess two electrons in a single MO, the 1s HOMO. For more information look in the Chemogenesis
webbook sections on Lewis
acids and Lewis bases and the Lewis
acid/base interaction matrix.