Slow Paramagnetic Relaxation of Some Pseudo-tetrahedral Ferrous Compounds in Zero Field

Slow (zero field)paramagnetic relaxation is reported for undiluted tetrahedral ferrous monomers for the first time. The Mossbauer spectra of Fe(2,9 di-CH3-phenanthroline)(NCS)2 and Fe(2,9 di-CH3 4,7 di-(j>-phenanthroline) (NCS)2 exhibit magnetic hyperfine splitting in the range 20 K to 1.6 K while the corresponding chlorides are rapidly relaxing paramagnets at all temperatures in zero field. Low temperature susceptibility and Mossbauer results indicate that D changes sign from positive to negative from the CI to NCS~ respectively with the foregoing ligands. Mossbauer spectra to 1.6 K confirm rapid paramagnetic relaxation for other pseudo-tetrahedral analogues (e.g. Fe(biquinoline)X2, X=C1, NCS). The slow relaxation is then peculiar to the 2,9 di-CH3~phenanthroline system.