Type 8 complexes form when a s-LUMO Lewis acid interacts with a pi-System Lewis base. Quite a number of Type 8 Lewis acid/base complexes are known, for example sodium cyclopentdienyl. However, as s-LUMO species are hard Lewis acids and delocalised pi-System Lewis bases are soft, it follows that the resulting species are mixed hard/soft complexes. For more information look in the Chemogenesis
webbook sections on Lewis
acids and Lewis bases and the Lewis
acid/base interaction matrix.