Density functional theory (DFT) using the finite cluster approach is utilized to compute binding energies, bond geometries, and vibrational properties of carbon monoxide adsorbed on Pt(111) as a function of the external interfacial field, focusing attention on the metal–CO bond itself. Comparison with electrode potential-dependent frequencies for the metal–CO (¿M–CO) as well as the much-studied intramolecular C---O (¿CO) vibration, as measured by in-situ Raman and infrared spectroscopy, facilitate their interpretation in terms of metal-chemisorbate bonding for this archetypal electrochemical system. Decomposing the calculated metal–CO binding energy and vibrational frequencies into individual orbital and steric repulsion components enables the role of such quantum-chemical interactions to the field- (and hence potential-) dependent bonding to be assessed. No simple relationship between the field(F)-dependent binding energies and the ¿M–CO frequencies is evident. While the DFT ¿M–CO–F slopes are negative at positive and small–moderate negative fields, reflecting the prevailing influence of back-donation, a ¿M–CO–F maximum is obtained at larger negative fields for atop CO, and a plateau for hollow-site CO. This Stark-tuning behavior reflects largely offsetting field-dependent contributions from p and s surface bonding, and can also be rationalized on the basis of changes in the electrostatic component of ¿M–CO from increasing M–CO charge polarization. A rough correlation between the field-dependent ¿M–CO frequencies and the corresponding bond distances, rM–CO, is observed for hollow and atop CO in that rM–CO shortens towards less positive fields, but becomes near-constant at moderate–large negative fields. A more quantitative correlation between the field-dependent C---O frequencies and bond lengths is also evident. In harmony with earlier findings (and unlike the ¿M–CO–F behavior), the ¿CO–F dependence is due chiefly to changes in the back-donation bonding component. The overall vibrational frequency-field behavior predicted by DFT is also in semi-quantitative concordance with experimental potential-dependent spectra.