Type 5 complexes form when s-LUMO Lewis acids interact with hydride ions to form saline hydrides. (The reaction actually proceeds via a redox reaction between the metal and hydrogen.) Saline hydrides Type 5 complexes may act as very strong bases (potassium hydride is one of the strongest bases available) which react with weakly acidic protons to form H2 or as hydride donor reducing agents. It turns out that for a number of subtle reasons organic chemists are more likely to regard and employ saline hydride reagents as proton abstracting bases, and inorganic chemists are more likely to regard and utilise them as reducing agents. For more information look in the Chemogenesis
webbook sections on Lewis
acids and Lewis bases and the Lewis
acid/base interaction matrix.