[Fe(btzp)3](ClO4)2 (btzp = 1,2-bis(tetrazol-1-yl)propane) represents the first structurally characterized Fe(II) linear chain compound exhibiting thermal spin crossover. It shows a very gradual spin transition (T1/2 = 130 K) which has been followed by magnetic susceptibility measurements and 57Fe Mössbauer spectroscopy. The structure has been solved at 200 and 100 K by single-crystal X-ray analysis. It crystallizes in the trigonal space group P3c1 with Z = 2 Fe(II) units at both temperatures. The molecular structure consists of chains running along the c axis in which the Fe(II) ions are linked by three N4,N4' coordinating bis(tetrazole) ligands. The main difference between the two forms appears to be in the Fe-N bond lengths, which are 2.164(4) A at 200 K and 2.038(4) A at 100 K. The Fe-Fe separations are 7.422(1) A at 200 K and 7.273(1) A at 100 K. The EXAFS results are consistent with the crystal structure. In both spin states, the FeN6 octahedron is almost regular within the EXAFS resolution. The Fe-N distance is found as 2.16(2) A at 300 K and 2.00(2) A at 40 K. The absence of the "7 A peak" in the EXAFS spectra of [Fe(btzp)3](ClO4)2, in contrast with what has been observed for the [Fe(4-R-1,2,4-triazole)3]-(anion)2 chain compounds, confirms that this peak can be used as the signature of a metal alignment only when it involves a strongly enhanced multiple scattering M-M-M path, with M-M spacing less than 4 A. Irradiation with green light at 5 K has led to the population of the metastable high-spin state for the iron(II) ion. The nature of the spin-crossover behavior has been discussed on the basis of the structural features.

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