The foregoing results show that the π-basicity of the Cu I sites in the Cu I M-MFU-4l (M = Zn, Mn, and Cd) series of metal-organic frameworks can be deliberately controlled by changing the radius of the central metal ion in the pentanuclear cluster node. Compared with Lewis acidic open metal sites in typical MOFs, π-basic open metal sites enable a higher magnitude of adsorption enthalpy than Lewis acidic metal sites for π-acidic gases. Consequently, the Cu I M-MFU-4l frameworks serve as rare examples of materials exhibiting a high isosteric enthalpy of H2 adsorption (-33.4, -26.9, and -22.7 kJ/mol for M = Zn, Mn, and Cd, respectively). Among the series, Cu I Cd-MFU-4l is only the second example of a MOF displaying an H2 binding enthalpy within the optimal range for ambient-temperature hydrogen storage. Additionally, the magnitude of the change in the adsorption enthalpy in the Cu I M-MFU-4l series is significantly larger than observed in several isostructural frameworks containing different metals or anions (Figure ). Note that the difference in enthalpy of the Cu I M-MFU-4l series arises from tuning the central metal ion of the pentanuclear cluster nodes-a metal center that is located three bonds away from the open Cu I sites that directly engage with H2. The series thus presents an unprecedented system for varying the isosteric enthalpy of H2 adsorption and therefore the hydrogen storage capacity of a material.