Mössbauer Hyperfine Interactions in Tetrahedral Fe(III) Ions

The 57Fe Mössbauer spectra have been measured for a series of compounds containing tetrahedral FeX4− anions, where X was Cl−, Br−, and NCO−. Two of them became antiferromagnets at low temperatures: NEt4FeCl4 (TN = 3.0°K) and NEt4FeBr4 (TN = 3.9°K). In an external magnetic field the latter compound showed a spin flop. The remainder were paramagnetic and magnetic hyperfine interactions were observed by applying a large magnetic field at low temperatures. The hyperfine field, | Hn |, was approximately independent of the cation R+ for FeCl4− and was about 470 kG. For the other ligands | Hn | decreased with increasing covalency of the Fe–X bond, being 420 and 394 kG for X = Br− and NCO−. The values of the isomer shift (approximately 0.3 mm/sec at 77°K relative to Fe) and | Hn | were lower than those of corresponding octahedral high-spin Fe(III) salts. Salts with large cations (AsPh4FeCl4) or large anions [NEt4Fe(NCO)4] showed a quadrupole splitting, and the sign of the electric field gradient (efg) in NEt4Fe(NCO)4 was positive.