The [Fe(etz)6](BF4)2 Spin-Crossover System-Part One: High-Spin ⇌ Low-Spin Transition in Two Lattice Sites

The [Fe(etz),](BF,), spin‐cross‐over system (etz = 1‐ethyl‐1 H‐tetrazole) crystallizes in space group P1, with the following lattice constants at 298 K: a 10.419(3), b=15.709(1), c = 18.890(2) Å = α = 71.223(9), β =77.986(10), and γ = 84.62(1)° V = 2862.0(9) Å3 and Z = 3. Two nonequivalent lattice sites, one without (site A) and one with (site B) inversion symmetry, are observed. The population of the two sites nA:nB is 2:l. Iron(II) on site A undergoes a thermal low‐spin (LS) → high‐spin (HS) transition with T1/2I, = 105 K. whereas that on site B remains in the high‐spin state down to cryogenic temperatures. Application of external pressure of up to 1200 bar between 200 and 60 K does not cause formation of the low‐spin state on site B. On site A the high‐spin state can be populated as a metastable state at 20 K by irradiating the sample with λ = 514.5 nm; on site B a light‐induced population of the low‐spin state can be achieved with λ = 820 nm. The fascinating electronic phenomenon of spin crossover in the [Fe(etz)6](BF4)2 system (etz = 1‐ethyl‐1H‐tetrazole) has been studied in detail by several experimental methods. A single‐crystal structure analysis is reported, which provides the basis for understanding the thermally, light‐ and pressure‐induced spin transitions in this system (shown schematically on the right).