The Schiff base ligand N1,N3-bis(3-methoxysalicylidene)diethylenetriamine (H2 valdien) and the co-ligand 6-chloro-2-hydroxypyridine (Hchp) were used to construct two 3d-4f heterometallic single-ion magnets [Co2 Dy(valdien)2 (OCH3 )2 (chp)2 ]⋅ClO4 ⋅5 H2 O (1) and [Co2 Tb(valdien)2 (OCH3 )2 (chp)2 ]⋅ClO4 ⋅2 H2 O⋅CH3 OH (2). The two trinuclear [Co(III) 2 Ln(III) ] complexes behave as a mononuclear Ln(III) magnetic system because of the presence of two diamagnetic cobalt(III) ions. Complex 1 has a molecular symmetry center, and it crystallizes in the C2/c space group, whereas complex 2 shows a lower molecular symmetry and crystallizes in the P21 /c space group. Magnetic investigations indicated that both complexes are field-induced single-ion magnets, and the Co(III) 2 -Dy(III) complex possesses a larger energy barrier [74.1(4.2) K] than the Co(III) 2 -Tb(III) complex [32.3(2.6) K].