A density functional investigation of the x-ray emission spectrum of NO adsorbed on Ru(001) has been carried out using model cluster calculations. The dipole matrix elements governing the emission probability were evaluated in the frozen ground-state approximation. The resulting simulated spectra exhibit all characteristic features of the experimental data. A detailed analysis of the electronic structure of the model clusters permits a complete rationalization of all observed trends. Furthermore, a picture of the surface chemical bond results in which the classical Blyholder frontier orbital model is extended to a three-orbital description for both the pi and sigma interactions. Comparison of different adsorption sites reveals that threefold coordinated NO features a stronger orbital interaction with the substrate than NO adsorbed in an on-top position.