High-resolution electron energy loss spectra (HREELS) of NO adsorbed at low temperature on the Ru(001) surface are reported with particular emphasis on the low coverage regime. The improved resolution compared to earlier studies allowed one to clearly separate the various vibrational bands and to establish correlations among them. The experimental data are analyzed with the help of linear combination of Gaussian-type orbitals local density functional model cluster calculations. We conclude that the loss peak of the law coverage samples at 1130 cm(-1) can be attributed to the stretching vibrations of upright oriented mu(3)-bridge nitrosyl species which are bound in an unusual configuration, i.e., via the oxygen atom. The dominating feature at small coverages around 1400 cm(-1), corresponding to the intramolecular vibration of NO moieties at the same site but bound via nitrogen, is calculated in agreement with experiment. Bent structures of the adsorbate are energetically less favored both for the mu(3)-ON and mu(3)-NO adsorption complexes. In contrast to the adsorption on the threefold hollow sites, NO molecules in the on-top position at Ru(001) bear a positive charge and vibrate at much higher frequencies. The importance of Pauli repulsion for the vibrational frequencies of adsorbed species is emphasized.