Based on the 4-substituted 1,2,4-triazole derivate ligand 4-benzene-1,2,4-triazole (L), a series of crystalline coordination complexes varying from mononuclear to trinuclear species, namely, [Zn(L)2Br2] (1), [Zn(L)2Br2] (2), [Fe(L)4(NCS)2]2 (3), [Fe2(μ2-L)3(L)2(NCS)4]·CH3OH·CH3CH2OH (4), [Fe2(μ2-L)3(L)2(NCS)4]·2CH3CH2OH (5), [Fe2(μ2-L)3(L)2(NCS)4]·2CH3CH2OH·1.5H2O (6), and [Ni3(μ2-L)6(L)4(H2O)2](NO3)6·15.5H2O (7), have been isolated. 1 and 2 present a temperature-induced polymorphic phenomenon of two zinc(II) coordination complexes with L. The solvent effect plays the key role for the self-assembly of these Fe(II) complexes 3–6: 3 contains mononuclear Fe(L)4(NCS)2 units without spin-transition behavior, whereas both 4 and 5 present binuclear Fe(II) complexes with three N1,N2-1,2,4-triazole bridges exhibiting incomplete spin-transition behavior. The low-temperature X-ray structural analysis (100 K) of 4 also confirms that one of the Fe(II) centers is located at the low-spin (LS) state and the other Fe(II) center is located at the high-spin (HS) state. Interestingly, when the binuclear Fe(II) complex 5 was exposed in the water atmosphere, solvent-induced single crystal-to-single crystal transformation can be observed, and the binuclear Fe(II) complex 6 exhibiting antiferromagnetic interactions can be isolated. Further, a trinuclear crystalline compound is isolated when Ni(II) salts were used to react with L. Variable-temperature magnetic susceptibility measurement (2–300 K) reveals antiferromagnetic interactions in 7. The polymorphic phenomenon (1 and 2), incomplete spin-transition phenomenon (4 and 5), and single crystal-to-single crystal transformation (from 5 to 6) also reveal great potential in the construction of these novel functional materials with L.