When a series of first-row M I transition metals is compared (Co I , Ni I , and Cu I ), the binding strength becomes stronger as we go left in the periodic table (Co I > Ni I > Cu I ). This can be rationalized by noting that less electronegative ions on the left side of the periodic table in the same row have more diffuse 3d orbitals that can show stronger orbital interaction with H2. It is worth noting that the trend in H2 binding strength for the series of 3d M I metals is opposite to that for typical M II metal sites, in which H2 binding strength becomes stronger as we move to the right in the periodic table (Mn II < Fe II < Co II < Ni II ) following the Irving-Williams series. Among the other 3d transition metals evaluated, Co I demonstrates excessively strong hydrogen binding, with the binding enthalpy marginally weakening from -45 kJ/mol to -41 kJ/mol when the central M II atom is altered from Zn II to Cd II . In turn, Ni I demonstrates more favorable but still too excessive binding enthalpies. Substituting Zn II with Mn II or Cd II shifts the binding enthalpy toward the optimal range, although the binding is still too strong. However, we note that the free energies of hydrogen binding for Ni I M II 4Cl3(bta)6 (M II = Mn, Cd) are comparable to those of Cu I M II 4Cl3(bta)6 (M II = Mn, Cd). This indicates that although the derivation of optimal binding enthalpy assumes the empirical enthalpy-entropy correlation, there are some cases where this correlation does not exactly hold, making it possible to realize optimal free energies with a larger magnitude of enthalpy. Even though the hydrogen binding enthalpies of Nicontaining clusters are too strong, the differences in adsorption entropy that depend on the identity of the M I atom shift the free energies toward the optimal region.