Cluster and periodic density functional theory (DFT) of carbon monoxide adsorbed atop on Pt (COads) show that ruthenium alloying weakens both the COads internal and C−Pt bonds and reduces the COads adsorption energy. A new theoretical model based on the π-attraction σ-repulsion is used to explain the above results. This model correlates (1) Mulliken population, (2) density-of-states analysis of the COads orbitals, (3) the individual interaction of these orbitals with the metal lattice bands, and (4) their polarizations within the COads molecule. In this study, the σ interaction has both attractive and repulsive components via electron donation to the metal bands and Pauli repulsion, respectively. Cluster DFT shows that the overall weakening of the COads internal bond upon alloying is due to the dominance of reduced σ donation to the metal (which weakens the COads internal bond) over increased π bonding between the carbon and oxygen. However, periodic DFT calculations show that both the σ donation and the COads internal π bonding are simultaneously reduced. The C−Pt bond weakening upon alloying is primarily due to increased exchange repulsion between the adsorbate and the substrate. The adsorbing Pt atom sp/d z 2 orbitals population increase upon alloying for both calculations.