Our combinatorial optimization reveals elongation of the H-H distance from that of the free H2 molecule (0.74 Å) in all clusters, thus implying π-backbonding interaction for activation of the H2 molecule, although the extent of the elongation varies. For a given peripheral M I site, the M I -H2 distance and the H-H distance are very similar with roughly 0.02-0.01 Å of differences, depending on the choice of the central M II ion. As expected from the comparison of the Cu I M4Cl3(bta)6 clusters, for a larger central M II ion, the M I -H2 distance becomes longer for a given M I , suggesting a weaker M I -H2 interaction. The weaker M I -H2 interaction for a larger M II central ion is also indicated by the degree of elongation of the H-H distance from that of free H2. Specifically, the larger central M II ion generally leads to less elongation for a given M I ion.