The bonding of CO molecules to single Au and Ag atoms on a NiAl surface was studied with a scanning tunneling microscope. A peak at 35 meV in the vibrational spectra of individual AuCO molecules is assigned to the hindered rotational mode. Vibrational spectra of individual AgCO molecules displayed peaks at 26 meV and 266 meV. These peaks are assigned to the hindered rotational mode and the C-O stretch mode, respectively. Scanning tunneling spectroscopy measurements show an unoccupied electronic resonance about 2.0 eV above the Fermi energy on single Au and Ag atoms. This resonance is quenched when monocarbonyls are formed due to electron donation from the CO ligand into the unoccupied electronic resonance. (C) 2003 American Institute of Physics.