Four Ni–Ln–Ni heterometallic complexes, [Ni₂LnL₂]NO₃·3H₂O (H₃L = tri(((3-methoxysalicylidene)amino)ethyl)amine, Ln = Gd for 1, Tb for 2 and Dy for 3, respectively) and [Ni₂DyL₂]ClO₄·MTBE·0.65H₂O (4, MTBE = methyl tert-butyl ether) have been synthesized by diffusion of methyl tert-butyl ether vapor into the reaction solution. The X-ray analyses demonstrated that the Gd^III ion in 1 exhibits rare seven-coordination, the Tb^III and Dy^III ions in 2–4 display unusual six-coordination, and two Ni^II ions and one Ln^III ion are bridged by six phenolato atoms to form linear Ni–Ln–Ni heterotrinuclear complexes for 1–4. All complexes exhibit weak ferromagnetic interactions between Ni^II and Ln^III ions. Alternating current susceptibility measurements demonstrated that compounds 3 and 4 behave as single-molecule magnets with the effective energy barriers of 14.17 and 11.13 K under zero direct current field. They are rare single-molecule magnets containing six-coordinate Dy^III ions.