We successfully synthesized $\left[\mathrm{Co}{\left(m\text{−}\mathrm{Py}\text{−}\mathrm{V}\right)}_3{\left({\mathrm{NO}}_3\right)}_2\right]$, in which a ${\mathrm{Co}}^{2+}$ ion adopts a seven-coordinate pentagonal bipyramidal (PBP) geometry. Molecular orbital calculations reveal that one of the three verdazyl radicals in the molecule forms a nonmagnetic singlet dimer through the strong intermolecular interaction, leading to a triangular spin necklace composed of the remaining spin-1/2 radicals and a spin-3/2 ${\mathrm{Co}}^{2+}$ ion. Magnetic susceptibility and specific heat measurements reveal contributions from ferromagnetic and antiferromagnetic exchange interactions, as well as a low-temperature phase transition. Electron spin resonance spectra and magnetization curves are consistently explained using a simplified model that treats the system as an anisotropic $S=3/2$ monomer coupled to an isotropic $s=1/2$ chain. Our results demonstrate that introducing PBP-coordinated transition-metal centers into radical-based complexes provides an effective route to incorporating strong magnetic anisotropy into designed quantum spin models, establishing a versatile platform for exploring frustrated and anisotropic triangular spin necklaces.

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• Frustrated magnetism
• High magnetic fields
• Magnetic anisotropy
• Magnetic interactions
• Magnetic phase transitions
• Magnetism
• Spin-orbit coupling
• 1-dimensional spin chains
• 1-dimensional systems