Three isostructural cyano-bridged 3d-4f compounds, [YFe(CN)(6)(hep)(2)(H2O)(4)] (1), [DyFe(CN)(6)(hep)(2)(H2O)(4)] (2), and [DyCo(CN)(6)(hep)(2)(H2O)(4)] (3), were successfully assembled by site-targeted substitution of the 3d or rare-earth ions. All compounds have been structurally characterized to display slightly distorted pentagonal-bipyramidal local coordination geometry around the rare-earth ions. Magnetic analyses revealed negligible magnetic coupling in compound 1, antiferromagnetic intradimer interaction in 2, and weak ferromagnetic coupling through dipolardipolar interaction in 3. Under an applied direct-current (dc) field, 1 (H-dc = 2.5 kOe, tau(0) = 1.3 x 10(-7) s, and U-eff/k(B) = 23 K) and 3 (H-dc = 2.0 kOe, t0 = 7.1 x 10(-11) s, and U-eff/k(B) = 63 K) respectively indicated magnetic relaxation behavior based on a single [Fe-III](LS) ion and a Dy-III ion; nevertheless, 2 (H-dc = 2.0 kOe, t0 = 9.7 x 10(-8) s, and U-eff/kB = 23 K) appeared to be a single-molecule magnet based on a cyano-bridged DyFe dimer. Compound 1, which can be regarded as a single-ion magnet of the [Fe-III](LS) ion linked to a diamagnetic Y-III ion in a cyano-bridged heterodimer, represents one of the rarely investigated examples based on a single Fe-III ion explored in magnetic relaxation behavior. It demonstrated that the introduction of intradimer magnetic interaction of 2 through a cyano bridge between Dy-III and [Fe-III](LS) ions negatively affects the energy barrier and chi(T) peak temperature compared to 3.