For CO, the metal-ligand charge transfer causes a decrease of the C-O bond order due to the antibonding character of the π * -LUMO orbitals. This is confirmed by the C-O bond distances being always larger for the CO adsorbed on site A (1.142 Å) than in site B (1.139 Å). The longer C-O bond distances can cause a red-shift of the stretching CO frequency with respect to the free CO. The stronger the backbond-ing, the weaker the C-O bond and the lower the stretching frequency. The computed C-O stretching frequency for gas phase CO appears at approx. 2123 cm -1 while upon adsorption on site A it is around 2092 cm -1 . The typical red-shift for a strong backbonding interaction is normally more than 100 cm -1 , e.g. the experimental shift for Cr(CO)6 is of ca. 143 cm -1 . As expected, the stretching frequencies of the CO located between the pyrazines remain unchanged upon adsorption consistent with the absence of backbonding. In the case of CO2, a backbonding interaction would increase the O-C-O bond distances due to the antibonding character of the π * -LUMO orbitals, resulting in lower stretching frequencies. Here, the O-C-O distances remain almost unchanged with respect to the free CO2 (1.172 Å versus 1.174 Å) and the symmetric and antisymmetric stretching frequencies shift by less than 10 cm -1 .