By using spin-polarized density function theory, we have established structural descriptors for CO vibrations on Fe(100) by structurally modeling the adsorbed CO with different occupation sites and orientations. The internal C-O bond length and the external C-Fe bond length can be taken as the structural descriptors for the variation of the vibration frequencies. Generally, the first thumb of rule embraces the relationship between frequency softness and orientation tilting from the surface normal. The breakdown of the simple rule of variations in chemical coordination as a measure of changes in adsorption energy is demonstrated for the adsorbate; the adsorption energy of the upright CO molecule at the bridge site serves as an exception that is contradict to the previous assumption that the adsorption with higher metal coordination gave rise to larger adsorption energy.