The optimized geometry of the model cluster without H2 reveals that the cluster having a larger central M ion planarizes the peripheral Cu I site, as indicated by a longer distance between Cu atom and the plane defined by three N2 atoms (d(Cu-N2 plane)) and by a larger angle between the central M, Cu, and N2 (∠M-Cu-N2) (Table ). Geometry optimization upon introduction of H2 at the Cu I site leads to Cu I moving away from the central M II ion by ~0.25 Å for all clusters (Figure ). This distortion is similar to what was reported in the previous study of a Cu I Zn4 cluster model. Therefore, the trend in the extent of Cu I pyramidalization across the Cu I M4 cluster models is also maintained upon H2 binding. The H2 molecule is predicted to be bound to the Cu I ion in a side-on manner, with the distance between the Cu atom and the H-H centroid, d(Cu-H2), Frequency analysis with anharmonic correction predicts ν(H-H) of 3251, 3317, and 3356 cm -1 for the Cu I Zn4, Cu I Mn4, and Cu I Cd4 clusters, respectively (Table ). The calculated ν(H-H) frequencies agree with the experimentally observed ν(H-H) signal to within 1% difference. We note that the frequency under the harmonic approximation predicts that a value of ν(H-H) in wavenumbers is roughly 10% larger than observed, implying that the anharmonic correction is essential for accurately modeling the vibration of H2 bound at a π-basic Cu I site. These calculations confirm that the shift of ν(H-H) for the H2 molecule bound to the Cu I ion follows Cu I Zn4 > Cu I Mn4 > Cu I Cd4 clusters, as observed via in situ gas dosing IR spectroscopy.