2-(2H-Tetrazol-5-yl)-1,10-phenanthroline (HL0), its alkyl-substituted derivatives (L-n, where n = 1 - 8, 10, 12, 14, and 16, denoting the carbon atom number of the alkyl chain) at the 2H position of the tetrazole ring, and their iron(II) complexes (a for [Fe(L-0)(2)], n a for [Fe(L-n)(2)](ClO4)(2), and nb for [Fe(L-n)(2)](BF4)(2)) were synthesized and characterized. The crystal structures of a, a center dot CH3OH, 1a center dot CH3OH, 1b center dot CH3OH center dot CH3CN, 2a center dot H2O, 2b center dot H2O, 4b center dot CH3OH, 5a center dot H2O, 5b center dot H2O, 6a, 6b, 7a, 7b, and 16a are described, along with thermal analyses. a undergoes an abrupt spin crossover (SCO) at 255 K with a hysteresis loop of 6 K. a center dot CH3OH, 2a center dot H2O, and 2b center dot H2O exhibit irreversible SCO behaviors due to the loss of solvent molecules upon heating. 3a, 3b, 4a, and 5a center dot H2O show simple spin transitions above 350 K. The desolvated samples of 4b center dot CH3OH and 5b center dot H2O undergo two-step spin transitions. 16a exhibits a two-step SCO behavior between 100 and 300 K, corresponding to sequential phase transitions from the low-spin (LS) phase to the intermediate phase and then to the high-spin phase, respectively, proved by crystal structure analysis and Fe-57 Mossbauer spectroscopy. 1a center dot CH3OH, 10a, 10b, 12a, 12b, 14a, 14b, and 16b show gradual and incomplete SCO behaviors after cooling down from 400 K. 1b center dot CH3OH center dot CH3CN, 6a, 6b, 7a, 7b, 8a, and 8b remain in the LS state even at 400 K. This proves that the alkyl side chains, together with the solvent molecules and anions, play a crucial role in the complicated SCO behaviors in this system.