A new rational approach has been developed for the synthesis of a mixed metal MIL-88B metal-organic framework based on a neutral mixed metal cluster, such as Fe(2)Ni(μ(3)-O). Unlike the conventional negative charged single metal cluster, the use of the neutral mixed metal cluster as nodes in the framework avoids the need of a compensating anion inside the porous MIL-88B system; thus the mixed metal MIL-88B becomes porous. The flexibility of the mixed metal MIL-88B can be controlled by terminal ligands with different steric hindrance. This allows us to reversibly customize the porosity of the MIL-88B structure at three levels of specific surface area as well as the pore volume.