The variations of the frequency differences of symmetric and asymmetric stretching vibrations in a series of carboxylato Fe(II) complexes have been theoretically studied. It is shown that structural information can be obtained from a direct comparison between the difference (Delta = nu(as) - nu(s)) in the asymmetric (v(as)) and symmetric (v(s)) carboxylate vibrations of the free anion and that of the coordinated species. The coordination mode approaches C-2 nu, symmetry as Delta decreases with respect to its value for the noncoordinated carboxylate. The use of IR spectroscopy in the resolution of speculated crystallographic structures is suggested.