Exchange-versus anisotropy-driven magnetism in phenoxido-bridged Ni(II) and Co(II) dimers

Two phenoxido-bridged dinuclear Ni(II) and Co(II) complexes, [M 2 (SAIDPPO) 2 (NO 3 ) 2 ] (M = Ni, Co), have been synthesized using a new Schiff base phosphine oxide ligand (SAIDPPO − ). Single-crystal X-ray diffraction revealed closely related Ni 2 O 2 and Co 2 O 2 cores with distinct magnetostructural features. Magnetic studies showed that the Ni(II) complex ( 1 ) exhibits ferromagnetic coupling ( J = +9.75 cm −1 ) associated with a near-orthogonal Ni–O–Ni angle (95.50°) and pronounced easy-axis anisotropy ( D = −19.44 cm −1 ). In contrast, the Co(II) analog ( 2 ) displays weak antiferromagnetic coupling ( J ≈ −4.32 cm −1 ) combined with significant easy-axis anisotropy ( D = −28.45 cm −1 ) arising from strong spin-orbit coupling. The comparison of the Ni 2 O 2 and Co 2 O 2 cores highlights how subtle structural variations, particularly the M–O–M angle, govern the balance between exchange interactions and magnetic anisotropy.