The mixed 3d-4f pentanuclear complex (Bu4N)[(Mn4YIII)-Y-III(shi)(4)(OAc)(4)(CH3OH)(4)]center dot CH3OH center dot H2O (1) (H(3)shi = salicylhydroxamic acid) was synthesized by the direct reaction of Y(NO3)(3)center dot 6H(2)O,, and H(3)shi. When an additional ligand, (NHBu3)(3)[W(CN)(8)]center dot 2H(2)O, was added, the mixed 3d-4f-5cl hexanuclear complex (Et4N)(8)[Mn-4(III) Y(shi)(4)(OAc)(4)Wv(CN)(8)] (WO4)(0.8) (2) was obtained. X-ray crystallographic analysis shows that the 3d-4f complex 1 represents a 12-metallacrown-4 (12-MC](4) structure, in which the metallacrown ring [Mn N-O](4) connection captures one Y-III ion with four bridging acetate anions, completing the eight-coordinated environment around Y-III ion, while four methanol molecules each coordinate to the Mn-III ions on the other side of the Y-III ion. After octacyanotungstate is introduced, the [Wv(CN)(8)] group substitutes for four methanol molecules of 1 to form complex 2. Magnetic studies indicate the overall antiferromagnetic coupling present within the MC ring of complex 1. However, interestingly, the dominant ferromagnetic coupling between Mnill ions was observed in complex 2. A susceptibility analysis shows that the natural spin alignments in 12-MC-4 metallacrowns are tuned from overall antiferromagnetic to dominant ferromagnetic fashions by magnetic coupling between Mnill ions and the W-V ion. Complex 1 [(Mn4YIII)-Y-III] retains an S = 0 ground state, and complex 2 [(Mn4YWV)-Y-III-W-III] shows obvious single-molecule magnet (SMM) behavior with an S-T = 11/2 ground state, respectively, before and after introduction of the octacyanotungstate group. The spin frustration geometrical structure constructed by four Mnin ions and one Wv ion was considered as the key factor for switching on the SMM properties of the 12-MC-4 system.