Cluster-model self-consistent-field studies of the core-level excitations of CO chemisorbed on Cu demonstrate several important features of the bonding for these states. We show that there are significant differences among the ligand-metal interaction energies for the C1s* and O1s* core excitations and for the ground state. We also discuss the relationship between these states and the screened ligand core-ion states. Finally, we show that different spin coupling of the open shells of the core-excited CO may be sufficiently large to be observable. These features are interpreted and explained in a qualitative way in terms of the equivalent core model. © 1985 The American Physical Society.