The [Fe(etz),](BF,), spin-cross-over system (etz = 1-ethyl-1Â H-tetrazole) crystallizes in space group P1, with the following lattice constants at 298 K:Â a10.419(3), b=15.709(1), c = 18.890(2) Ã…, Î±Â = 71.223(9),Â Î²Â =77.986(10), andÂ Î³Â = 84.62(1)Â° V = 2862.0(9) Ã…3Â andÂ ZÂ = 3. Two nonequivalent lattice sites, one without (site A) and one with (site B) inversion symmetry, are observed. The population of the two sitesÂ nA:nBÂ is 2:l. Iron(II) on site A undergoes a thermal low-spin (LS)Â â†’Â 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 withÂ Î»Â = 514.5 nm; on site B a light-induced population of the low-spin state can be achieved withÂ Î»Â = 820 nm.