A combined experimental and theoretical study of carboxylate coordination modes: a structural probe.

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 (nu(as)) and symmetric (nu(s)) carboxylate vibrations of the free anion and that of the coordinated species. The coordination mode approaches C(2v) 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.