Is the frequency of the internal mode of an adsorbed diatomic molecule a reliable guide to its local adsorption site?

Although the C-O stretching frequency for the system Ni>{111}c(4>x2)-CO falls in the so-called bridge region, a structure determination with photoelectron diffraction shows that the CO molecules are actually adsorbed in threefold hollow sites. These, and other recent data, seriously call into question the practice of adsorption site assignment solely on the basis of the stretching frequency of an internal mode. © 1993.