A review of the literature shows that vibrational spectroscopy has been extremely successful in anticipating the structures Of C-2 chemisorbed species that are later evaluated by the more direct methods of photoelectron diffraction (PED), low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM). More difficulties have attended these comparison procedures as applied to the spectra of adsorbed CO and NO, where the diffraction methods have shown that on (1 1 1) planes the 3-fold bridged species on 'hollow' sites occupy a wider range of coverages than had been anticipated spectroscopically. Exceptional coverage dependence of the positions of the VCO and vNO absorption bands leads to overlap of the characteristic wavenumber ranges for the 2-fold bridge and 3-fold bridged surface species except at low coverages. The high coverage-dependence of these spectra is associated with the exceptionally high intensity of these absorptions leading to vibrational dipolar coupling and to sensitivity to static electronic interactions with neighbouring adsorbed molecules. Such factors are minimal in the spectra of the C-2 species. It is concluded that the 3-fold bridged CO surface species has an extended wavenumber range 1950-1800 cm(-1) (high to low coverage), which overlaps at moderate coverages with that of the 2-fold species between 2000 and 1870 cm(-1). An exhaustive literature survey has yielded improved vNO wavenumber ranges for adsorbed NO species at low coverages, namely M-NO linear (on-top site), 1800-1720 cm(-1); M-NO bent, 1700-1620 cm(-1); 2-fold bridge, 1570-1500 cm(-1); 3-fold bridge (hollow site) 1470-1380 cm(-1).