Two new complexes belonging to the family [Fe-II(L)(2)(NCX)(2)] have been synthesized and structurally characterized. Complexation of the ligand L-O = 2,5-di(2-pyridyl)-1,3,4-oxadiazole results in the formation of two derivatives of [Fe-II(L-O)(2)(NCS)(2)] (1) with the common s-trans and the rarely observed s-cis conformation. No thermal spin crossover (SCO) was observed for the amorphous bulk material of the mixture. Using the new ligand L-Se = 2,5-di(2-pyridyl)-1,3,4-selenadiazole, compound [Fe-II(L-Se)(2)(NCS)(2)]center dot 1.4DCM center dot 0.6MeOH (2 center dot 1.4DCM center dot 0.6MeOH) was structurally characterized by single crystal X-ray diffraction. Bulk material of [Fe-II(L-Se)(2)(NCS)(2)]center dot MeOH (2 center dot MeOH) exhibits a thermally induced SCO with small hysteresis [T-1/2(down arrow) = 91 K, T-1/2(up arrow) = 96 K]. LIESST and reflectivity studies have been performed on the SCO complexes [Fe-II(L-S)(2)(NCS)(2)], [Fe-II(L-S)(2)(NCSe)(2)], [Fe-II(L-S)(2)(NCBH3)(2)]center dot H2O [L-S = 2,5-di(2-pyridyl)-1,3,4-thiadiazole], and 2 center dot MeOH. All complexes belong to the T-0 = 90 K line [T(LIESST) = T-0 - 0.3T(1/2)]. [Fe-II(L-S)(2)(NCS)(2)], that exhibits a two-stepped thermal SCO process, has been found to also exhibit two well-separated T(LIESST) temperatures [T(LIESST, 1) = 44 K; T(LIESST, 2) = 53 K].