The adsorption of CO and surface chemistry of Si are well-understood topics in surface science. However, research into the adsorption of CO on the Si(111)-7 x 7 surface is deficient. In this study, the adsorption of CO on Si(111)-7 x 7 is investigated via high-level density functional theory calculations using cluster model. Two adsorption configurations are found to be kinetically and thermodynamically viable: on-top on rest-atoms and back-bond insertion on adatoms, both binding to the surface via C atom. Structural, electronic, and spectroscopic properties of the adsorbates indicate a sigma-donating/pi-accepting nature of the CO Si bonds in both configurations. The domination of sigma-donation in the on-top configuration results in a net positive charge on the on-top adsorbate, and the opposite situation yields a net negative charge on the back-bond insertion adsorbates. Our study provides a detailed understanding of the previous experimental observations of fundamental surface chemical phenomena, suggesting possible applications of Si surface functionalization using CO. (C) 2017 Elsevier B.V. All rights reserved.