Infrared spectra of two spin-transition polymers, [Fe(Htrz)(3)](ClO4)(2) and [Fe(NH(2)trz)(3)](ClO4)(2), were investigated. In the structure, the metal centers are bridged through nitrogen atoms in the 1- and 2-positions of the three triazole ligands, each iron being sixfold coordinated to nitrogen. For comparison, Fourier transform infrared (FT-IR) spectra of a similar, but permanently low-spin compound, [Fe(Htrz)(3)]Cl-2, and a permanently high-spin compound, [Fe(Htrz)(3)]SO4, were recorded. Mid-infrared spectra indicate bidentate bridging of triazole Ligands. Far-infrared spectra of low-spin [Fe(Htrz)(3)](ClO4)(2) and [Fe(Htrz)(3)]Cl-2 indicate six equal Fe-N bonds for the low-spin coordination sphere. Far-infrared spectra of high-spin [Fe(Htrz)(3)](ClO4)(2), [Fe(NH(2)trz)(3)](ClO4)(2), and [Fe(Htrz)(3)]SO4 indicate weakening of all six Fe-N bonds in the high-spin state, two of the bonds being weakened to a lesser degree than the other four. A low- to high-spin transition is observed for [Fe(Htrz)(3)](ClO4)(2) upon lowering the pressure to 110 mbar. The low-temperature high- to low-spin transition of [Fe(NH(2)trz)(3)](ClO4)(2) was investigated by far-infrared spectroscopy. (C) 2000 Elsevier Science Ltd. All rights reserved.