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.