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Three Co II complexes, [Co 2 (H 2 L 1 ) 2 ](ClO 4 ) 4 ·4CH 3 OH (1), [Co 2 (H 4 L 2 ) 2 ](ClO 4 ) 4 (2) and [Co 4 (H 4 L 2 ) 4 ](ClO 4 ) 8 (3), were constructed by the self-assembly of the symmetrical dihydrazone ligands H2L1 and H4L2 with Co II ions under different synthetic conditions. The fish-bone-like complex 1 was obtained using the ligand H2L1 in methanol via the solvothermal method, while the self-assembly of H4L2 with Co II ions is solvent-dependent, producing the fish-bone-like complex 2 and [2 × 2] grid-like complex 3. Magnetic susceptibility measurements and theoretical calculations reveal that the large negative D values for the three complexes stem from their easy-axis magnetic anisotropy. Ac magnetic susceptibility measurements disclosed field-induced slow magnetic relaxation behaviors and the presence of Raman and/or direct processes of the three complexes at various applied dc fields.