Mössbauer spectroscopy of stereochemically non-rigid and related organometallic molecules
Abstract
The Mössbauer parameters for cyclo-octatetraene iron tricarbonyl [COTFe(CO)3] and a number of related iron-tricarbonyl complexes have been determined both for the neat solids and for frozen solutions using solvents which set to a “glassy” matrix. The quadrupole splitting parameter is found to be diagnostic with respect to distinguishing between the ligand symmetry of the hydrocarbon framework around the Fe(CO)3 moiety. The Mössbauer data for COTFe(CO)3, in conjunction with new low temperature n.m.r. data on a number of iron-tricarbonyl complexes, indicate that the cyclo-octatetraene ring acts as a 1,3-diene in solution at low temperatures, in consonance with the bonding information derived from X-ray diffraction data on the neat solid. Moreover, the usual isomer shift-quadrupole splitting correlation diagram for these absorbers shows that the Mössbauer measurements can serve as a useful bridge relating structural information derived from measurements on solids and those requiring liquid or solutions samples. Detailed measurements of both the 57Fe and 119Sn resonances in [π-C5H5Fe(CO)2]2SnCl2 and π-C5H5Fe(CO)2SnCl3, in glassy matrices as well as the neat solids, show that low lying optical modes which are normally observed in the far infra-red (10–200 cm–1) region of the spectrum can be studied by Mössbauer methods. That such optical modes contribute to the Mössbauer-Lamb recoil-free fraction is deduced from the nonconstancy of the normalized area ratio for the two resonance spectra in different matrices, in consonance with a simple Debye model for the lattice dynamics of the Mössbauer atom.