The adsorption of CO on metal surfaces is represented by molecular orbital cluster models of CO at an on top site and adsorbed normal to the surface carbon end down. Ab initio SCF and MCSCF calculations are performed for several clusters. The new constrained space orbital variation CSOV approach is used to analyze the bonding and to compare CO adsorption on Al, representative of sp metals, with that on Cu, representative of transition metals. There is a large repulsion between the superposed free CO and metal charge distributions which is considerably smaller for Cu than for Al because there are fewer valence a electrons for Cu than for Al. The CSOV analysis shows that the metal to CO tt donation is much more important than the CO to metal a donation. It is also shown that for Cu, the dir contribution to the metal ir donation is larger than the valence 4pir contribution. The dir donation is compared between Fe, Ni, and Cu and this donation and the metal-CO interaction are found to be different in the order Fe > Ni > Cu. © 1984, American Vacuum Society. All rights reserved.