The spin-transition complexes [Fe(Htrz)3](ClO4)2 and [Fe(NH2trz)3](ClO4)2 I. FT-IR spectra of a low pressure and a low temperature phase transition

Abstract Infrared spectra of two spin-transition polymers, [Fe(Htrz) 3 ](ClO 4 ) 2 and [Fe(NH 2 trz) 3 ](ClO 4 ) 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 ]SO 4 , were recorded. Mid-infrared spectra indicate bidentate bridging of triazole ligands. Far-infrared spectra of low-spin [Fe(Htrz) 3 ](ClO 4 ) 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 ](ClO 4 ) 2 , [Fe(NH 2 trz) 3 ](ClO 4 ) 2 , and [Fe(Htrz) 3 ]SO 4 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 ](ClO 4 ) 2 upon lowering the pressure to 110 mbar. The low-temperature high- to low-spin transition of [Fe(NH 2 trz) 3 ](ClO 4 ) 2 was investigated by far-infrared spectroscopy.