Four new complexes [Ni 3( μ-L) 6(H 2O) 6](NO 3) 6·6H 2O ( 1), [Co 3( μ-L) 6(H 2O) 6](NO 3) 6·6H 2O ( 2), [Ni 3( μ-L) 6(H 2O) 4(CH 3OH) 2](NO 3) 6·4H 2O ( 3), [Co 3( μ-L) 6(H 2O) 4(CH 3OH) 2](NO 3) 6·4H 2O ( 4) (L = 4-amino-3,5-dimethanyl-1,2,4-triazole) were synthesized and structurally characterized by X-ray single-crystal diffraction. The structural analyses show that complex 1 and 2 are isomorphous; complex 3 and 4 are isomorphous. Four complexes all consist of the linear trinuclear cations ([M 3( μ-L) 6(H 2O) 6] 6+ (M = Ni,Co) for 1 and 2; [M 3( μ-L) 6(H 2O) 4(CH 3OH) 2] 6+ (M = Ni,Co) for 3 and 4), NO 3 − anions and crystallized water molecules. In the trinuclear cations, the central M(II) ions and two terminal M(II) ions are bridged by three triazole ligands. Other eleven solid solution compounds which are isomorphous with complex 3 and 4 were obtained by using different ratio of Ni(II) and Co(II) ions as reactants and ICP result indicates that ligand L has higher selectivity of Ni(II) ions than that of Co(II) ions. The magnetic analysis was carried out by using the isotropic spin Hamiltonian Ĥ = −2 J( Ŝ 1 Ŝ 2 + Ŝ 2 Ŝ 3) (for complexes 1 and 3) and simultaneously considering the temperature dependent g factor (for complexes 2 and 4). Both the UV–Vis spectra and the magnetic properties of the solid solutions can be altered systematically by adjusting the Co(II)/Ni(II) ratio.