Transition metal(II) fluoroborates and nitrates give various complexes with 1,2,4-triazole. Most of these complexes are expected to be polynuclear. The infrared and ligand field spectra have been used to predict some structural aspects of the compounds. The iron complex shows a low spin → high spin transition on heating.

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Fe3(Ettrz)6(H20)6](CF3S03)6 (Ettrz = 4-ethyl-1,2,4-triazole) is a linear trinuclear coordination compound, in which the two pairs of metal centers are each linked by three 1,2-bicoordinating triazole ligands. The terminal coordination sites are occupied by H20 molecules. Upon cooling, the central iron(II) undergoes a high-spin to low-spin transition. Móssbauer and magnetic susceptibility data show that this spin crossover gradually takes place between 230 and 190 K. The structural consequences of the spin crossover have been studied by crystal structure determinations at 300 and 105 K. The compound crystallizes in the space group Pile, with two trinuclear molecules per unit cell. The lattice constants are a = b = 14.488 (5) Á and c = 19.858 (5) Á at 300 K and a = b = 14.265 (7) Á and c = 19.632 (2) Á at 105 K. Fe-O(water) distances at 300 K are 2.156 (4) Á, whereas the Fe-N distances are 2.157 (4) Á (terminal Fe) and 2.174 (4) Á (central Fe). The Fe-L distances of the terminal iron hardly change (0.01-0.02 Á) upon cooling. However, the observed decrease in the Fe-N bond lengths (about 0.14 Á) upon cooling for the central Fe(II) ion reflects a clear strengthening of these bonds in the low-spin state. The low-temperature magnetic susceptibility and Móssbauer data indicate that no magnetic exchange exists between the terminal iron(II) ions.

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