The interaction of CO molecules with metal surfaces involves the interplay between the highest occupied molecular orbital (HOMO) 5σ and the lowest unoccupied molecular orbital (LUMO) 2π* of the CO molecule and the metal d-states . In this context, Pauli repulsion arises from the overlap between the metal d-electrons and the CO σ-orbitals, causing a repulsive interaction that weakens the overall bonding. Strong intra-and inter-molecular bonding must offset this repulsion to enable CO* adsorption, even if the overall metal-CO bond remains weak .