Thermally and optically induced spin transition effect on the structure ofiron(II) polymeric complexes

Polymeric iron(II) complexes of the composition Fe(atrz)(3)A(2), where atrz = 4-amino-1,2,4-triazole, A = anions, have been synthesized and characteri zed. The compounds exhibit an abrupt thermally driven spin transition (S=0 - S=2) at temperatures varying from 175 K to 400 K depend on composition. X AFS measurements were performed at the VEPP-3 storage ring at the Budker In stitute of Nuclear Physics in Novosibirsk. In all studied compounds the Fe- N and Fe-C interatomic distances increase upon the thermally induced transi tion to the high spin (HS) state by amounts from 0.12 Angstrom to 0.24 Angs trom. The existence of long-life optically excited metastable HS states was established for the Fe(atrz)(3)(ClO4)(2) at 4.2 K. The changes in structur e upon the transition to a metastable HS state differ from those in the the rmally induced spin transition.