Molecular motion in solids. The Mössbauer spectra of (η-cyclopentadienyl)(η-fluorobenzene)iron(II) salts

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Abstract
The 57Fe Mössbauer spectra of [Fe(η-FC6H5)(η-C5H5)]A (A = PF6, AsF4, SbF6 or BF4) have been recorded at various temperatures in the range 77–350 K. That of the hexafluorophosphate salt is a quadrupole doublet which broadens above 228 K, coalesces at 240 K, and is fully narrowed at 300 K. This is interpreted in terms of isotropic reorientation of the cation. Fitting of the spectral results by an appropriate model yields an activation energy of 19.6 kJ mol–1 for this process. X-Ray diffraction results obtained at 300 K indicate a primitive cubic cell of side 6.9 Å with totally disordered cations. There is a first-order phase transition at 225 K which heralds the onset of rotation. The AsF6 – and SbF6 – salts display analogous behaviour but the spectrum of the BF4 – salt is a simple quadrupole doublet over the entire temperature range.