Through middle-temperature solvothermal reactions of CoCl2 center dot 6H(2)O with the rigid-angled ligand 3-(2'-pyridyl)-5(4 ''-pyridyl)-1,2,4-triazole (Hdpt24), we obtained the three cobalt complexes {[Co(dpt24)(2))](3)center dot 4DMF center dot 1.5H(2)O}(n) (1), {[Co(dpt24)(2))](2)center dot H2O}(n) (2), and Co(dpt24)(2)(Hdpt24)center dot H2O] (4) at N,N-dimethylformamide (DMF)/H2O volume ratios of 9:1, 1:1, and 0:1, respectively. Interestingly, 1 underwent transformations into 2, {[Co(dpt24)(2)]center dot 0.5DMF}(n) (3), and 4 when treated with DMF/H2O at volume ratios of 1:1, 1:9, and 0:1, respectively. Moreover, 3 and 4 converted back to 1 in 9:1 DMF/H2O and to 2 in 1:1 DMF/H2O; 3 transformed into 4 in H2O and vice versa in 1:9 DMF/H2O. Structurally, 1 is a three-dimensional (3D) 2-fold interpenetrating distorted NbO-type complex, 2 possesses a two-dimensional layer metal-organic framework, 3 is a 3D 2-fold interpenetrating typical NbO-type complex, and 4 is a wheel-shaped mononuclear neutral complex. This approach, using a mixed solvent's component ratio to direct the syntheses and conversions of four cobalt complexes, provides unprecedented control for crystal engineering.