A family of four Ln(III) complexes has been synthesized with the general formula Ln (NO ) (L) (S) (Ln = Gd, Tb, Er, and S = H O; , and , respectively/Ln = Dy, S = MeOH, complex ), where HL is the flexible ditopic ligand '-(1-(pyridin-2-yl)ethylidene)pyridine-2-carbohydrazide. The structures of isostructural MeOH/H O solvates of these complexes were determined by single-crystal X-ray diffraction. The two Ln ions are doubly bridged by the deprotonated oxygen atoms of two "head-to-head" 2.21011 (Harris notation) L¯ ligands, forming a central, nearly rhombic {Ln (μ-OR) } core. Two bidentate chelating nitrato groups complete a sphenocoronal 10-coordination at one metal ion, while two bidentate chelating nitrato groups and one solvent molecule (H O or MeOH) complete a spherical capped square antiprismatic 9-coordination at the other. The structures are critically compared with those of other, previously reported metal complexes of HL or L¯. The IR spectra of - are discussed in terms of the coordination modes of the organic and inorganic ligands involved. The f-f transitions in the solid-state (diffuse reflectance) spectra of the Tb(III), Dy(III), and Er(III) complexes have been fully assigned in the UV/Vis and near-IR regions. Magnetic susceptibility studies in the 1.85-300 K range reveal the presence of weak, intramolecular Gd ∙∙∙Gd antiferromagnetic exchange interactions in / = -0.020(6) K based on the spin Hamiltonian = -2 ( ∙ ) and probably weak antiferromagnetic Ln ∙∙∙Ln exchange interactions in - . Ac susceptibility measurements in zero dc field do not show frequency dependent out-of-phase signals, and this experimental fact is discussed for in terms of the magnetic anisotropy axis for each Dy center and the oblate electron density of this metal ion. Complexes and are Single-Molecule Magnets (SMMs) and this behavior is optimally observed under external dc fields of 600 and 1000 Oe, respectively. The magnetization relaxation pathways are discussed and a satisfactory fit of the temperature and field dependencies of the relaxation time was achieved considering a model that employs Raman, direct, and Orbach relaxation mechanisms.