The Mössbauer effect and chemistry. Part 9. Molecular motion in solids. The spectra of (η-cyclohexatriene)(η-cyclopentadienyl)iron(II) hexafluorophosphate

Abstract

The ⁵⁷Fe spectrum of the title cationic sandwich compound has been recorded at various temperatures in the range 4.2–350 K. At low temperatures it consists of a simple quadrupole doublet and a magnetically perturbed spectrum at 4.2 K reveals that V_zz is positive. At T 273 K additional absorption to low energy is observed. As the temperature is increased a four-peak spectrum gradually develops which has an inner quadrupole doublet of magnitude approximately half that of the outer doublet. The spectrum is fully developed at 321 K but further heating brings about an inward collapse onto a single peak of much reduced intensity. This critical temperature is associated with a phase change of the solid. These processes are reversible. A model has been developed to account for the variation of spectrum with temperature. The organometallic cation is assumed to lie at an eight-co-oridnate site made up of eight PF₆ groups. At low temperatures the symmetry axis of the cation is collinear with the Z axis of the tetragon. As the temperature is raised, the X and Y orientations are Boltzmann populated and jump reorientations X↔Y take place at a significant rate giving rise to the inner doublet. These reorientations become faster with increasing temperature and on further heating those from Z↔X and Z↔Y become significant on the Mössbauer time scale and the spectrum becomes broadened and confused. Finally, all reorientations are so rapid that the cation is effectively spherical: this brings about a catharsis for the tetragonal phase in the form of a transition to one of cubic symmetry and the Mössbauer spectrum collapses to a narrow single peak.