The atom and symmetry specific properties of x-ray emission spectroscopy have been applied to the investigation of CO adsorbed on Ni(100) and Cu(100) surfaces. In comparison to ab initio electronic structure calculations, obtained in density functional theory, we develop a consistent electronic structure model of CO adsorption on transition and noble metals and extend to a conceptual model of the surface chemical bond. A strong CO-substrate interaction is found, characterized by significant hybridization of the initial CO orbitals and the metal bands. In the pi system an allylic configuration is found as the result of orbital mixing between the CO 1 pi, 2 pi* and the metal d(pi)-band which is manifested experimentally in the observation of an oxygen lone-pair state. In the sigma system experimental evidence of equally strong orbital mixing has been found. Energetically, the adsorbate-substrate complex is stabilized by the pi-interaction but is destabilized by the sigma-interaction. Furthermore, the internal C-O bond carried by the pi-interaction is weakened upon adsorption, which is opposite for the internal C-O sigma bond that is strengthened. The equilibrium properties of CO adsorbed on these metals are found to be the direct result of the balance between the sigma-and pi-interactions; both in terms of the total energy and the local bond properties.