The Sabatier principle states that the binding strength between reactants and catalysts should not be very strong to avoid overbinding and poisoning effects, nor very weak to give enough time for diffusion and collision. In addition, the products should interact weakly with the surface to guarantee a facile desorption. In Figure , pure Pt 4 CO presents the largest CO binding energy (isomer I, BE[CO] = -3.13 eV), suggestive of a tendency to undergo CO poisoning. Nonetheless, adding a single Ge atom in Pt 4 GeCO, the BE[CO] is reduced to -2.63 eV; and as the Ge concentration increases a gradual drop in the BE[CO] is observed. This trend is in line with previous observations in gas-phase and graphene-supported PtGe nanoclusters. The bonding between Pt and CO can be understood in terms of the Blyholder model. Briefly, it describes the interaction with an electron transfer from the 5σ of CO to the empty Pt d orbitals (donation step), and an electron transfer from the Pt d orbitals to the empty 2π* of CO (back-donation step). According to the Bader analysis (Figure ) and projected density of states (PDOS, Figure ), there is a net charge transfer from the cluster to CO 2π*. As a result, the C-O bond is weakened and elongated slightly from 1.14 Å in gas-phase to ~1.17 Å.