Inelastic neutron scattering spectra of the complexes [M2M'O(OOCCH,),(OH,)$+, where M2M' = Cr:", Fe!" and Fe:"Fe", are reported and assigned in comparison with infrared and Raman data.
VIBRATIONAL spectra of complexes of the general type [M30(OOCR)6L3] (Fig. 1) have been reported in detail in a series of papers beginning in 1981 [ 11. The complexes include homonuclear trivalent metal clusters (M = Cr, Mn, Fe, Rh and Ru), mixed-metal M2M' and mixed-valence M:"M"' systems. They are important model systems for the study of magnetic and electronic interactions between metal ions and we have already found that for a proper understanding of such phenomena a detailed knowledge of the vibrational characteristics of these compounds is essential. One point of major importance has been the characterization of the mixed-valence compounds as "localized" or "delocalized" relative to the time-scale of molecular vibrations. Our approach to this problem has been to assign the normal modes and hence the symmetry, of the metal ion triangle, by comparing suitable examples of mixed-valence and mixed-metal clusters. In particular, the in-plane vibration v,(M30) of the central oxygen atom has been used to show that (in the case of M= Fe and Mn), the electron transfer process M"-,M"' is slow on the vibrational time scale [ lc, e]. To proceed further, we plan to obtain data on the curvature of the relevant adiabatic potential surfaces, rather than relying on simple theoretical models which are all we have at present [lc], and for this a knowledge of vibrational couplings is required. We have already invoked coupling to explain relative intensity changes in the n(M,O) region of the i.r. spectra, but the effects are too small and the assignments too uncertain to give any futher information.
Up to now, all our vibrational studies of these systems have used i.r. and Raman spectroscopies. The strengths and limitations of these techniques are well known: especially the fact that observed band frequencies can be used relatively easily in the estimation of the force field in which the molecule vibrates, while the observed intensities do not easily provide such information.
Inelastic neutron scattering spectroscopy provides different, and complementary information. An incoherent inelastic neutron scattering (IINS) spectrum is a measure of the amplitude-weighted density of vibrational states. When an observed IINS band can be identified with a normal vibrational mode, the intensity is a function of the scattering cross-sections and amplitudes of motion of the atoms involved [2]. In particular, the IINS spectrum is dominated by the motions of hydrogen atoms, when these are present, firstly because 'H has the highest incoherent scattering cross-section and secondly because being the lightest atom, it often has the largest amplitude of motion. On the other hand, each peak in the internal vibrational density of states which apears in the IINS spectrum has associated with it a series of lattice phonons whose intensity may cause excessive broadening. Furthermore, the