We present a study of the metal-centered vibrations in the first step of the Fe(III)-catalyzed Michael reaction. Nuclear inelastic scattering of synchrotron radiation was carried out on a shock-frozen solution of FeCl3 center dot 6H(2)O in 2-oxocyclopentane ethylcarboxylate (CPEH), as well as on the solid reference compounds FeCl3 center dot 6H(2)O, [N(CH3)(4)][FeCl4], and Fe(acac)(3). In addition to the vibrations of the FeCl4- anion at 133 and 383 cm(-1), a multitude of modes associated with the complex Fe(CPE)(2)(H2O)(2) could be identified. Normal-mode analysis on different isomers of the simplified model complex Fe(acac)(2)(H2O)(2) as well as that of the full complex carrying two entire CPE ligands was carried out using density functional calculations. Comparison with experiment suggests that the facial bis(diketonato) isomer probably dominates in the reaction mixture. Thus, we have identified for the first time the isomeric structure of an iron-based intermediate of a homogeneous catalytic reaction using nuclear inelastic scattering.