Anomalous spin crossover of mechanically strained iron(II) complexes, with 1,10-phenanthroline with their counterions, NCS^- and PF₆/^-

Effects of counterions on the change in the magnetic behavior due to mechanical amorphization have been examined on iron(II) complexes with 1,10-phenanthroline, Fe(phen)₂(NCS)₂, [Fe(phen)₃](NCS)₂·H₂O, and [Fe(phen)₃](PF₆)₂, at temperatures between 2 K and 300 K. Amorphization results in the increase in χ(M)T, where χ(M) is molar magnetic susceptibility and T is temperature, in two modes, i.e., a jump at temperatures below 20 K and a subsequent gradual increase. The first mode is attributed to the mechanically induced high spin state (⁵T(2g)) which is made metastable below 20 K. The second is the thermally induced transition from a low spin (¹A(1g)) to a high spin (⁵T(2g)) state for the distorted complexes with the widened distribution of the ligand field strength. The stabilization of the HS state is mainly due to the ligand field distortion caused by mechanical amorphization. Coordination of ionic species, NCS^- and PF₆/^-, originally serving as counterions, plays a significant role in the ligand field distortion. (C) 2000 Academic Press.