High-spin α low-spin transition in [Fe(NCS) 2(4,4′-bis-1,2,4-triazole) 2](H 2O). X-ray crystal structure and magnetic, mössbauer and EPR properties

Abstract

The Fe II ion in [Fe(NCS) 2(4,4′-bis-1,2,4-triazole) 2](H 2O) has distorted tetragonal symmetry with two trans-oriented NCS − ligands. It shows a very abrupt high-spin α low-spin transition at 123.5 K on cooling and 144.5 K on warming. The compound is rather stable in vacuo at room temperature; however, samples which have passed the spin transition once lose their water of hydration above 240 K in vacuo. The dehydrated substance does not show a spin transition; it is high spin in the whole temperature range. Mössbauer ligand-field spectra and magnetic behaviour of both the hydrated and non-hydrated compounds are discussed. The spin transition has been followed by EPR measurements with the aid of traces of Cu 2+ ions which could be substituted for Fe 2+ in the tetragonal structure. In the high-spin phase the EPR signal is very broad and featureless; in the diamagnetic low-spin phase it is very sharp and resolved in hyperfine and superhyperfine structures. This unusual method to follow the spin transition was shown to be quite generally applicable. In the crystal structure of Fe(NCS) 2(C 4H 4N 6) 2(H 2O) the distances FENCS are 2.125(3) Å, and FeN(ligand) 2.180(3) and 2.188(2) Å. The water molecule is connected to the non-coordinating ligand N-atom by hydrogen bonding.