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A structural, magnetic and Mössbauer spectroscopic study of an unusual angular Jahn-Teller distortion in a series of high-spin iron(II) complexes.

Single crystal X-ray structures and susceptibility data are described for six homoleptic iron(II) complex salts, of 2,6-di(pyrazol-1-yl)pyridine or a 3,3"-disubstituted derivative of it. Zero field Mossbauer spectroscopic data for four of the complexes, and one previously reported analogue, are also discussed. Four of these compounds exhibit an unusual angular Jahn-Teller distortion towards C(2) symmetry to differing degrees, while the other two exhibit structures close to the "ideal" D(2d) symmetry for this ligand set. This structural distortion has two components: a twisting of the plane of one ligand relative to the other about the N{pyridine}-Fe-N{pyridine} vector, so that the two ligands are no longer perpendicular; and a rotation of one ligand about the Fe ion, so that the N{pyridine}-Fe-N{pyridine} angle < 180 degrees. Susceptibility data show that all the complexes are fully high-spin between 5 and 300 K, but yield an unusually wide range of zero-field splitting parameters for the different compounds of between 2.6 and 13.4 cm(-1). Magnetostructural correlations suggest that a low value of |D| is diagnostic for a high degree of "rotation" distortion. The Mossbauer spectra imply that an increased quadrupole splitting might also be diagnostic for the presence of the angular distortion.